JP3317807B2 - Expert system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an expert system having a knowledge base and an inference engine.

[0002]

2. Description of the Related Art Conventionally, expert systems for control problems have been used in the fields of aircraft, plants, power plants, and the like.
It is used for interpreting and diagnosing the current situation from input information such as sensor information and drafting a plan for operations and countermeasures corresponding to the situation.

In general, an information processing process in control is constituted by a cycle of situation recognition-plan drafting. That is, the state of the control target or the current situation where the control target is placed is recognized from the input data, and an action plan to be taken based on the recognized result is drafted. Here, the recognition process is a selection problem in which the listed solution space is selected by searching, whereas the plan formulation process is a synthesis problem in which solutions are generated so as to satisfy the constraints. Therefore,
The control problem is largely divided into two parts, a recognition process and a plan formulation process.

For this reason, as shown in FIG. 2, a conventional control-type expert system is composed of a situation recognition unit 20 for making a situation, interpretation and diagnosis, and a plan making unit 21 for making an operation and a control plan. Inference processing is performed for each processing element. The situation recognizing unit 20 performs situation recognition of each subsystem, failure diagnosis of equipment, and the like based on knowledge of an expert, a knowledge base in which information such as a configuration of an object and a physical rule is stored. In addition, the plan drafting unit 21 drafts an action plan such as optimal measures and operations based on the result of the situation recognition unit 20 based on the knowledge similarly stored in the knowledge base.

[0005] However, the conventional control-type expert system has a feature that the processing is complicated because it is composed of a very large number of elements such as devices and control devices.
On the other hand, in many cases, there is a real-time constraint that inference must be performed within a predetermined time.

On the other hand, there is a control-type expert system in which a situation recognition unit and a plan drafting unit are subdivided for each component such as each control device and subsystem, and distributed processing is performed to increase the processing efficiency. Is being developed.

However, in the method of subdividing the situation recognition process and the plan formulation process, it is not possible to completely divide each of the distributed processing units into problems, so that there is a possibility that there is a contradiction between the planned plans. is there. For this reason, it is necessary to detect inconsistencies in each of the plans and resolve conflicts.

[0008] The inconsistency between the plans is established when a change in the state of the system that is expected when a certain plan is executed does not satisfy the constraints of another plan. However, in the conventional system, the following processing is performed as a method for deriving the relationship between the plan execution result and the constraint condition.

(1) At the stage of planning, all problems are inferred to generate a consistent plan.

(2) The relationship between the state change of the system and the constraint conditions when the plan is executed is stored in advance in the form of rules in the knowledge base used in the plan drafting process as much as possible.

[0011]

In the conventional control-type expert system, the above-mentioned methods (1) and (2) for detecting inconsistency between generated plans and resolving conflicts include the following.
There are the following problems.

(1) When detecting inconsistencies with other plans at the stage of planning a plan, all situations must be considered in the end, the processing becomes extremely complicated, and inference is performed within a predetermined time. You may not be able to do it. For this reason, in a system having a very large time constraint such as a control system of a nuclear power plant or a flight management system, there is a possibility that a very dangerous situation involving human life may be caused.

(2) The change in the state of the system when each plan is executed involves all the components of the system and is very diversified. In addition, there is a problem that the state of the system changes with time, and it is not possible to accumulate information on all relationships in the knowledge base in advance.

An object of the present invention is to detect inconsistencies between generated plans and resolve conflicts, perform inference processing at high speed, and acquire all knowledge about the relationship between constraints and target states in advance. An object of the present invention is to provide an expert system that does not need to be stored in a knowledge base in the form of rules.

[0015]

In order to achieve the above-mentioned object, an expert system according to the present invention comprises a structure information storing means for storing a structural model representing a structural relationship of an object, and a causal relationship between objects as constituent elements. A causal relationship storing means for storing a causal model representing the relationship, a recognizing means for recognizing a current situation and identifying a cause of failure with respect to the input initial event, and a recognizing means for each of the events identified by the recognizing means. Based on the results, draft plan contents such as appropriate operations and countermeasures, constraints on the execution of the plan contents, changes in the target situation when the plan contents are executed, and priorities When there are a plurality of events for which a plan has been created by the plan creating means, knowledge about the structure of the target stored in the structure information storage means, Based on the knowledge about the causal relationship stored in the engagement storage means, execute the planned plan by judging whether or not the change in the situation of the target when executing each plan satisfies the constraints of other plans and changing conditions of the target when the leads to relationships between constraints other plans, and conflict detection means for detecting the presence or absence of conflict between plan, in contradiction detection means, when a conflict is detected, inconsistency Of the plan with the lower priority
Select lower priority alternative plan instead of plan
And repeat the process of detecting the presence or absence of inconsistency again.
And conflict resolution means for resolving conflicts between plans ,
Resolve conflicts between the plans and execute all plans that have been drafted.

[0016]

According to the present invention, plan candidates are drafted by plan drafting means completely decentralized for each plan object, and when there are a plurality of drafted plans, inconsistencies between these plans are detected and collectively determined. This is to resolve the conflict.
Therefore, compared to the conventional method of inferring all problems in advance and generating a consistent plan in the plan drafting means, the processing in the plan drafting stage can be simplified, and the inference processing can be speeded up. be able to.
Therefore, it is possible to use the expert system for controlling an aircraft or a power plant that must perform inference within a predetermined time, thereby increasing the safety of the aircraft or the power plant. be able to.

Further, when detecting the presence or absence of inconsistency between a plurality of planned plans, when the plan constraints and the plan are executed based on the structural model representing the target structural relationship and the causal model representing the causal relationship. In order to derive the relationship with the target state change, it is not necessary to store in advance the knowledge about the relationship between the constraint condition and the target state in the knowledge base in the form of rules.

[0018]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an expert system according to a first embodiment of the present invention.

Referring to FIG. 1, an expert system according to the present embodiment includes a recognizing unit 11 and a plan preparing unit 12.
A contradiction detecting unit 13, a conflict resolving unit 14, a man-machine interface 15, a data input unit 16, a structure information storing unit 17 storing a structural model representing a target structural relationship, and A causal relationship storage means 18 for storing a causal model representing a causal relationship between certain objects.

The recognizing means 11 comprises a knowledge base 11a in which knowledge on situation recognition and fault diagnosis is stored, and an inference engine 11b for performing situation recognition and diagnosis based on the knowledge. Exists.

The plan drafting means 12 is composed of a knowledge base 12a in which knowledge on countermeasures, operation procedures, controls and the like in an emergency is stored, and an inference processing section 12b for drafting a plan based on the knowledge. Similar to 11, multiple plans with multiple priority levels
The number is generated and sent to the conflict resolution means 14.

The structure information storage means 17 stores knowledge about a target structure as a structure model. The structural model is
Is controlled a relationship or a kind It is a structured representation of the hierarchy of objects represented by of relationships.

The causal relationship storage means 18 is provided in the structure storage means 1.
The causal relationship between the plurality of objects represented by 7 is stored as a causal model. The causal model expresses, for each object, an object name, a possible state of the object, a qualitative relationship with other objects, and the like in a frame format.

The inconsistency detecting means 13 includes the structure information storing means 1
Based on the knowledge of the structure stored in the storage unit 7 and the knowledge of each component stored in the causality storage unit 18, a search is made for inconsistencies between the drafted plans.

The conflict resolving means 14 comprises the plan making means 12
When a new plan is proposed, the inconsistency detecting means 13 determines the presence or absence of inconsistency, and selects an optimal plan combination without inconsistency.

FIG. 3 is a block diagram showing a configuration of a flight expert system in an airplane according to a second embodiment of the present invention.

The flight support expert system 39 of the present embodiment includes a data management device 36, a recognition unit 30, a plan planning unit 31, a conflict resolution unit 32, a structural model 34,
Causal model 35, inconsistency detector 33, and integrated display device 3
7.

The data management device 36 includes a data input device 38a for inputting flight plan information, communication information, etc., a barometric altimeter 38b, an atmospheric speedometer 38c, a fuel weight meter 38d,
Various data are input from the GPS 38e and the engine speed counter 38f.

The recognizing unit 30 is divided for each problem area, such as recognizing and recognizing an engine state, recognizing and recognizing a hydraulic power system, recognizing a navigation system, recognizing weather conditions, and the like.
Based on various sensor data such as speed, altitude, engine speed, fuel, radar information, flight plan information, communication information, etc. inputted via the data management device 36, failure diagnosis and grasp of the situation for each target problem. I do.
The information on the diagnosis result and the situation inferred by the recognizing unit 30 is then sent to the plan drafting unit 31.

The plan drafting unit 31 drafts a response plan such as a control command, an operation, and advice based on the information. The plan drafting unit 31 is distributed over problem areas such as engine control, hydraulic system control, and landing support, and, like the recognition unit 30, drafts a plan for each target problem. The plans drafted by the plan drafting unit 31 are collected by the conflict resolution unit 32.

In the conflict resolution section 32, the plan drafting section 31
Accumulate all the plans sent from them according to their priority,
Manage and select consistent plan combinations based on the concepts of priority and fulfillment added to each plan. At this time, in order to determine whether the newly proposed plan and the plan adopted so far are in conflict, the system status change and the constraint conditions at the time of executing each plan are transferred to the inconsistency detection unit 33. I do.

The inconsistency detecting section 33 derives the relationship between the situation change of each plan and the constraints based on the structural model 34 and the causal model 35 relating to the aircraft, and determines whether or not the constraints are satisfied. Thus, the presence or absence of inconsistency between the plans made is detected. Target of inconsistency detection
The plan that is not required is managed by the conflict resolution unit 32 as an alternative plan.
Is managed. When the conflict-resolving section 32 selects an optimal plan without contradiction, the plan is executed by displaying information on the display 38g via the integrated display device 37 or by sending a command to the actuator 38h.

Next, examples of the plan, the structural model 34 and the causal model 35 made by the plan making section 31 are shown below.

An example of an expression of a plan in the flight assistance expert system 39 will be described based on a plan for dealing with oil leakage of the lubricating oil of the left engine shown in FIG.

The plan is described in a frame structure composed of slots as shown in FIG. The NAME slot represents the plan name, and the AGENT slot represents the name of the plan drafting department that generated this plan.

A plan has a priority (PRIORITY) indicating how important the goals of the plan are. The priority is a value set for each goal of the plan, and is an index for a problem of which plan is given priority when plans conflict. For example, the goal of addressing engine lubricant leaks is much more urgent than the goal of correcting flight course deviations. If there is a conflict between the two plans, priority must be given to dealing with engine lubricant leaks.

When there are a plurality of plans, the plan drafting section 31 proposes all the plans. Some of the plans are completely satisfactory for the goals, while others are less desirable. In this flight support expert system 39, the value indicating the degree of satisfaction of the plan with respect to the target is expressed as SA
TISFACTION, the concept of sufficiency, is used. The value set in this slot indicates the degree of satisfaction with respect to the target in percentage.

The weight of each plan is represented by the product of the priority of the target and the degree of satisfaction with the target. These sufficiencies and priorities serve as indices for resolving plan conflicts.

In the plan, a state change of how an object constituting the aircraft changes as a result of executing the plan is represented by a slot called EFFECT expressed by a pair of an object and its attribute. . In this example, the left engine is turned off as a result of executing the plan.

The constraint of this plan is CONSTRAINT
Described in the slot. This is also expressed as an object-attribute pair like EFFECT. In this example, the constraint of this plan is that the total engine thrust must be normal.

The specific coping operation is described in the last ACTION section. If this plan is selected as a result of conflict resolution, AC
Execute the coping operation described in the TION section. In this example,
Shutdown of the left engine, single-engine flight of the right engine, and emergency landing assistance are described.

FIG. 5 shows an example of a structural model 34 relating to an aircraft. The structural model 34 describes concepts related to various aircraft systems, the external environment, and flights. a relationship or a
kind It is structured as a hierarchy of objects represented by of relationships, and is represented by a tree structure as shown in FIG.

FIG. 6 shows an example of a causal model 35 relating to an aircraft. In the causal model 35, each object is linked to other objects via parameters.

FIG. 7 shows a specific expression example of each object of the causal model 35. An object is described in a frame structure as shown in FIG. 7, and each slot has an object name (NAME) and a higher-level concept (IS
A), information such as a possible state (STATE) of the object is shown. The attributes described in the STATE slot include:
Preconditions for the attribute and parameters that affect it are added. In FIG. 7, the pump is turned on when the parameter of the number of revolutions of the left engine is xx rpm, and as a result, the hydraulic pressure of the left hydraulic power system (Hydraulic F
luid Pressure 1) is normally obtained (AVAILABLE). Qualitative relationships between objects are represented by CHANGE slots, QUALITY slots, and EFFECT slots. The CHANGE slot qualitatively describes the influence of the change in the state of the object on the parameter. In the example, the change in the hydraulic pump is equal to the qualitative change in the hydraulic pressure. The QUALITY slot represents a qualitative relationship between the parameters associated with this object,
Here, it is shown that the parameter having a positive effect on the hydraulic pressure is the engine speed. Also, EF
The FECT slot is the name of the object that affects or is affected by this object, along with its parameters. In FIG. 7, the hydraulic pump 1 (Hydraulic Pump 1) of the hydraulic power system has the engine speed (R
It is shown to be affected by the left engine via the Evolution, and to control subsystems such as Ailerons via the left hydraulic pressure.

Next, the processing procedure for conflict resolution in the conflict resolution unit 32 will be described.

FIG. 8 is a flowchart showing the procedure of the conflict resolution process.

In the plans generated by the plan drafting unit 31, there are a plurality of countermeasure plans which usually have different sufficiencies as countermeasures for the same situation. For example, there are several plans for coping with a decrease in the amount of lubricating oil of the left engine, such as stopping the engine, suppressing the output of the engine, and observing the situation. The conflict resolution unit 32 first determines the degree of satisfaction, STATISFACTION,
One plan with the highest slot value is selected (step 41). Next, there is no conflict between this plan and other plans.
Ika, the conflict with other plans is resolved by the conflict resolution unit 32
Is it by sending the values and constraints EFFECT slots plan that is transferred to run in the inconsistency detecting unit 33 determines whether plan conflicts (step 42, 43).
If you find that both plans are not competing,
The newly entered plan is adopted.

When a conflict is found between the plans, the higher-priority plan described in the PRIORITY slot of both plans is prioritized, and the lower plan is replaced with a lower-satisfaction alternative plan. (Steps 43, 44,
45), the constraint condition is determined again (step 42). If there is no alternative plan in the low-priority plan, the higher-priority plan is similarly replaced with the alternative plan and determined (steps 43, 44, 46, 47), and a plan in which no conflict occurs is found. Repeat this until If there is a conflict between the plans to the end, a plan with a high degree of satisfaction of both plans is presented to the pilot (step 4).
8) Execute the selection by asking for instructions.

Next, the operation of this embodiment will be specifically described.

The aircraft is called the Hydraulic System,
There are dual redundant hydraulic power systems that transmit pilot operations to the aircraft actuator subsystem by hydraulic pressure. Here, these are referred to as left and right hydraulic power systems. There are one left and right pumps for supplying the hydraulic pressure, and they are operated by the engines on each side. If one of the hydraulic power systems fails to function properly, the other hydraulic power system can supply hydraulic pressure to the defective hydraulic system by controlling a valve called a transfer valve.

If the hydraulic pump of the right power system fails due to any cause and stops, in order to maintain the control of the machine normally, the transfer valve is operated to supply the hydraulic pressure to the hydraulic system on the defective side. Can be considered. However, this countermeasure for failure of the right hydraulic pump has a constraint that the left hydraulic power system operates normally.

At the same time, it is assumed that the oil pressure drops due to oil leakage in the lubricating oil system of the left engine, which may cause a risk of engine fire. The most effective response to engine oil leaks is to shut down the engine to prevent engine fire. The best solution when considering only this problem is to shut down the left engine. This is because even if one engine is stopped, thrust enough to fly with the remaining engines can be obtained.

However, since this countermeasure does not consider the influence on other factors, there is a possibility that inconsistency may occur when viewed comprehensively. For example, in this case, if you take this measure, by stopping the left engine,
The pump on the left hydraulic power system stops. For this reason, since the right hydraulic pump is broken, both the left and right redundant hydraulic power systems are stopped, and the control of the aircraft becomes ineffective.

As described above, there is a possibility that a conflict may occur between a plurality of plans proposed by the plan drafting unit 31 based on the constraint conditions of the plans. This makes it impossible to execute all of the proposed plans simultaneously.

Therefore, the operation of the flight support expert system of this embodiment for canceling the plan conflict will be described below.

The operating status of the right hydraulic pump is input via the data input device 38a and is constantly monitored by the hydraulic power system recognition unit. When the information of the sudden stop of the pump is input via the data input device 38a, the recognizing unit 30 determines that the stop of the pump in flight is an abnormal situation, and the oil of the hydraulic power system and the left engine are normal. Therefore, it is diagnosed that the cause is a failure of the pump itself.

Based on the result, the inference engine regarding the hydraulic power system of the plan drafting section 32 makes a plan to move the transfer valve to supply the hydraulic pressure on the left side to the hydraulic system on the right side. .
FIG. 9 shows a simplified representation of this plan. In this plan, the hydraulic pressure on the left is above the operating limit (Hydraul
ic Fluid Pressure 1> 2000 psi).

Next, when the situation of the leakage of the lubricating oil of the left engine is recognized by the inference engine relating to the engine system of the recognition unit 30, the plan planning unit 32 of the engine system is recognized.
Then, based on this result, the plan shown in FIG. 10 is generated and sent to the conflict resolution unit 32.

The conflict resolving unit 32 takes out a plan with a high degree of sufficiency, that is, an engine lubrication oil reduction plan 1 (hereinafter, plan 1) shown in FIG. Then, data is sent to the inconsistency detection unit 33 in order to determine whether or not the contents of the EFFECT slot in this plan and the constraints of other plans do not conflict.

In the left engine stop plan, a state change that the Left Engine object is turned off is described in the EFFECT slot. In the inconsistency detecting unit 33, the structural model 3
4 and an object called Left Engine from the causal model 35, and the change in the engine speed is led from this CHANGE slot. Engine speed is Air Speed and FuelPr
Influenced by essure, even if the engine is stopped, it rotates slightly due to rotation based on speed. Inference part is QUAL
Based on this qualitative causal relationship described in the ITY slot, the engine speed when the engine is stopped is inferred. Next, the fact that the change in the engine speed affects the hydraulic pump 1 is derived from the contents of the EFFECT slot. The intelligent cockpit system then references the hydraulic pump 1 object. Here, the pump is turned off by the engine speed lower than the EFFECT slot, and CHANGE
From the content described in the slot, it is derived that the change in the pump from on to off is equal to the qualitative change in the oil pressure, and thus the oil pressure decreases due to the decrease in the engine speed. Since the oil pressure is affected only by the engine speed from the QUALITY slot, a specific value of the oil pressure with respect to the engine speed is inferred.

Finally, the inconsistency detecting section 33 sets the hydraulic pressure on the left side to 0 (H
ydraulic Fluid Pressure = 0) However,
This conflicts with the constraints in the plan for dealing with a right hydraulic pump failure, and it is discovered that there is a conflict between the two plans.

The conflict resolution unit 32 determines whether there is an alternative in the plan with the next lowest priority, that is, plan 1, and replaces it with plan 2 for engine lubricating oil reduction (hereinafter plan 2). Plan 2 has a 7 engine output
Although it is shown that it is suppressed to 0%, this does not completely eliminate the possibility of bearing seizure due to a decrease in the oil pressure of the lubricating oil. However, 70% of the engine output is a value that allows the left hydraulic power pump to operate normally, and the constraints of the right hydraulic pump handling plan are satisfied. Therefore, the system understands that there is no conflict between this plan 2 and the right hydraulic plan coping plan, and adopts this plan 2. Nuclear power plant control systems,
Systems with real-time processing such as aircraft flight management systems
In a system, each time, depending on the changing situation,
Action is required. For example, in an aircraft
Engine failure, the engine management
The recognizing unit 30 recognizes the failure and sets a response plan for the failure.
A plan drafting section 31 for engine management drafts. Through
Such situations always occur at a predetermined timing
It does not occur, it occurs at unexpected timing
Therefore, a plan is generated in an event-driven manner. And a new program
Each time a run candidate is generated, the plan is
Sent to the conflict resolution unit 32,
Resolves conflicts with

[0063]

As described above, according to the present invention, first, a plan is prepared by completely decentralized plan preparation means, and the conflict resolution is collectively performed on a plurality of plans prepared. This has the effect of simplifying the processing in the plan drafting stage and speeding up the inference processing. In addition, when detecting the presence or absence of inconsistency among a plurality of generated plans, the structure of the target Based on structural models that represent relationships and models that represent causal relationships,
In order to guide the relationship between the constraints of the plan and the state change of the target when the plan is executed, there is no need to store in advance the knowledge about the relationship between the constraints and the target state in the form of rules in the knowledge base, There is an effect that knowledge loss, error, and redundancy can be avoided, and a knowledge base can be easily constructed.

[Brief description of the drawings]

FIG. 1 is a block diagram of an expert system according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional expert system.

FIG. 3 is a block diagram showing a configuration of a flight assistance expert system according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a representation example of a plan.

FIG. 5 is a diagram showing an expression example of a structural model 34;

FIG. 6 is a diagram illustrating an expression example of a causal model 35;

FIG. 7 is a diagram showing a specific expression example of each object in the causal model 35.

FIG. 8 is a flowchart illustrating a processing procedure for conflict resolution.

FIG. 9 is a diagram showing an example of expressing a plan for a failure of the right hydraulic pump.

FIG. 10 is a diagram showing a representation example of a plan for lubricating oil leakage of the left engine.

[Explanation of symbols]

 Reference Signs List 10 inference processing unit 11 recognition means 11a knowledge base 11b inference engine 12 plan drafting means 12a knowledge base 12b inference engine 13 inconsistency detection means 14 conflict resolution means 15 man-machine interface 16 data input means 17 structure information storage means 18 causal relation storage means 19a Target 19b Operator 20 Situation Recognition Unit 30 Recognition Unit 31 Plan Planning Unit 32 Conflict Resolution Unit 33 Conflict Detection Unit 34 Structural Model 35 Causal Model 36 Data Management Device 37 Integrated Display Device 38a Data Input Device 38b Barometric Altimeter 38c Atmospheric Speed Meter 38d Fuel Weight 38e GPS 38f Engine tachometer 38g Display 39h Actuator 39 Flight support expert system 41-48 steps

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-175849 (JP, A) JP-A-6-259255 (JP, A) JP-A-4-186434 (JP, A) 273337 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G06F 9/44

Claims (2)

(57) [Claims] 1. An expert system, comprising: a structural information storage unit storing a structural model representing a structural relationship of an object; a causal relationship storing unit storing a causal model representing a causal relationship between objects as respective constituent elements; A recognition means for recognizing a current situation and identifying a cause of a failure with respect to the identified initial event, and a plan content such as an appropriate operation and a countermeasure based on a result of each event identified by the recognition means. A plan creating means for creating a plan candidate consisting of constraint conditions when executing the plan contents, a change in the target situation when the plan contents are executed, and a priority, and a plan is created by the plan creating means. If there are a plurality of events, the knowledge about the structure of the object stored in the structure information storage means and the causality stored in the cause-effect relation storage means Inconsistency that detects whether there is any inconsistency between the plans that have been planned by determining whether the change in the target situation when executing each plan satisfies the constraints of other plans based on the knowledge about the staff detection means, if the conflict in the conflict detection means is detected, inconsistency
Among the plans with lower priority,
Instead, select a lower priority alternative plan and try again.
By repeatedly detecting the presence or absence of inconsistency,
An expert system comprising conflict resolution means for resolving conflicts between programs, and executing all plans created by resolving conflicts between plans. 2. The conflict resolving means selects one plan having a high degree of satisfaction, which indicates a degree of satisfaction with a target, from a plurality of planned plans. If there is no conflict between the plan and another plan, or if a conflict between the plan being executed and the conflict being resolved by the conflict resolution means is found by the conflict detection means, If there is an alternative plan in the lower plan, replace the lower priority plan with a less satisfactory alternative plan, determine again whether or not there is a conflict, and replace it with the higher priority plan 2. The method according to claim 1, wherein if there is a plan, a plan that does not cause a conflict is found by repeating the process of replacing the plan with a higher priority with an alternative plan with a higher degree of satisfaction and repeatedly determining whether or not both plans conflict. Expert system.