TITLE: TRAP FOR AIR CONDITIONERS FIELD: The invention relates to devices used for trapping liquids flowing through pipes, and 5 also for using the liquid within the device to physically block odours otherwise drawn through drain pipes into ventilation apparatus including air conditioners and heat pumps for heating or cooling spaces. BACKGROUND The underlying problem is that air conditioning units which include heat exchanger 10 means and a fan to draw air over or through the heat exchanger will, from time to time, generated water from condensation on to the heat exchanger. This water is collected into a tray beneath, and in installations larger than domestic versions the water is carried through a downwardly sloping pipe to an outlet preferably a drain. Typically the heat exchanger is contained in a box and negative pressure may be developed within, from 15 time to time. It is customary to provide a water trap along the drain. Retention of some of the water passing through the trap, coupled with a shape that forces air flow below a surface of retained water at a localised low point in the trap creates a seal that prevents sewer gas from passing back and through the air conditioner ducts into the occupied space of the building. 20 Traps for use with the condensation drains from ducted air conditioners are generally made in the style of a "P" trap from PVC pipe elbows, bends or like parts glued together on the site. It is necessary that water is always present in such traps, because of a requirement to form a seal. The P-trap concept is widely used in plumbing and for draining other equipment especially where the drain channel is closed and leads to a 25 sewer. Without sufficient water, the water seal does not function. Traps can and do dry out, and poor venting can siphon or blow water out of the traps. This is usually avoided by venting the drain pipe to the atmosphere downstream of the trap so that the drain lines never operate at a pressure much higher or lower than atmospheric pressure. 1 Prior-art P traps are assembled by gluing from standard pipe connectors such as right angle bends and T junctions made of PVC. The parts are opaque. They require cutting apart and re-gluing for inspection and maintenance, and it is not possible to see the water inside the trap, or to conveniently open and clean the trap, or replace or refill the 5 required water. One prior-art solution resembles a glued-together trap and includes a curved transparent portion to receive a visible amount of water with a cap through which a bottle brush can be inserted in order to displace any contamination. But for that device, the contamination is displaced, not removed. Therefore, the problem to be solved may be stated as "to provide a replacement for a P 10 trap in which the trapped water is visible and may be replenished without difficulty". OBJECT An object of the present application is to provide an improved trap having the qualities of displaying the amount of liquid or contaminants within a reservoir to a person, and of permitting cleaning or refilling the reservoir, or at least to provide the public with a 15 useful choice. SUMMARY OF INVENTION In a first broad aspect, the invention provides a trap for use alongside air conditioner apparatus having a draining tray wherein the trap comprises a housing including attachment means for (a) a container located below the housing; (b) an inlet pipe from 20 the draining tray opening into a first internal chamber leading into an internal tube directed down toward the base of the container, (c) an outlet pipe to a drain opening into a second internal chamber open to the interior of the container and open to an air vent located above the housing to an adjacent atmosphere; the internal dropper pipe comprising a trap when in use and providing that water is present within the container; 25 said trap comprising an obstruction for gas at a low pressure differential between the inlet pipe and the outlet pipe. In a related aspect, the container comprises a re-attachable, transparent bowl located beneath the housing and serving as a reservoir for trapped water 2 Preferably the container is fixed to the housing using a screw thread which is sealably clamped by compression of an included O-ring washer, in order that it does not become detached because of vibration. Alternatively the container is fixed in place using a bayonet lock which is sealably 5 clamped by compression of an included O-ring washer, in order that it does not become detached because of vibration. Preferably the container is replaceable and includes a separate sealably attachable lid, so that, when in use, a contaminated reservoirs may be taken off the trap, sealed with the lid, and removed. 10 In a second related aspect, the inlet pipe enters the trap along the same axis as that of the outlet pipe leaving the trap. Optionally the inlet pipe enters the trap at a higher height than the height at which the outlet pipe leaves the trap. Alternatively the entire trap is mounted with a slight downward slope towards the outlet 15 pipe. Preferably the housing is moulded from a plastics material and the reservoir is formed from a transparent material, selected from a range including plastics and glass that is resistant to a disinfectant substance selected from a range including a solution of hypochlorite. 20 Optionally the replaceable container can serve as a collection point for testing for the presence of pathogenic organisms in a public health scheme. PREFERRED EMBODIMENT The description of the invention to be provided herein is given purely by way of 25 example and is not to be taken in any way as limiting the scope or extent of the invention. 3 Throughout this specification unless the text requires otherwise, the word "comprise" and variations such as "comprising" or "comprises" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 5 DRAWINGS Fig 1: is an oblique view of an exterior of a trap according to the invention. Fig 2: is an elevation view of the inlet side of the trap. Fig 3: is a perspective view of a preferred reservoir. Fig 4: is a cross-section through the body 101 of a preferred trap, showing inlet, drop, 10 breather, and outlet pipes in place, and sufficient water inside the reservoir. EXAMPLE 1 The conventional type of ducted air conditioning machine for which this trap is designed includes a motorized suction fan drawing room air from ducts over heat exchanger coils carrying refrigerant fluid within, past the fan, and out again through 15 ducts and back to the area being air conditioned. The fan and coils are usually contained within a box. Some air may be freshly drawn from outside. When the air makes contact with the heat exchanger coils, condensation of water may occur, depending on relative humidity and temperature. This water falls down and is collected in a drip tray beneath the heat exchanger, and then carried through a drop pipe and preferably through a trap 20 to the exterior. Volatile materials dissolved in the dripped water may be carried back to the area being air conditioned. There is typically a slight negative pressure inside the box, and the trap including its air leak must allow excess water to flow from the drip tray to a drain yet prevent reverse flow of air, as will be described. Figs 1, 2 and 3 show the physical form of one version of a complete trap 100 according 25 to this invention. A housing 101 which is a solid part made of a plastics material such as a polyvinyl chloride (PVC) and shaped by any suitable methods known to those 4 skilled in the art, (including machining, extrusion, injection moulding, casting and combinations of those methods) includes attachment means 102 for an inlet pipe 109 of typically 26-27 mm outside diameter and commonly known in the relevant arts as a "drop pipe". The likely and preferred means of attachment is by glueing the outside of 5 the pipe to the interior of the entry to the first of two internal chambers 109A. The apertures receiving the inlet pipe and the outlet pipe are slightly tapered - at perhaps 1 degree, to provide a positive grip on the standard plastics pipe. Nevertheless it is best to secure the trap to the pipes with a suitable glue, for reliability. The only outlet from chamber 109A comprises the down-feeding internal pipe 112 (Fig 10 1, Fig 4) which extends into the cavity 106 inside the bowl 107, though not so close to the floor of the bowl that flow will be restricted, that forms the actual vapour trap if the bowl includes some water. In Figs 1, 2 and 4, bowl 107 has been screwed into the recess underneath the housing 101. Fig 2 is an end elevation, showing proportions. The housing around the second internal chamber 11 0A, which is continuous with the interior 15 of the bowl, includes attachment means 103 for a similar outlet pipe 110 also of typically 26-27 mm outside diameter, (c) attachment means 108A for an approximately vertical breather pipe 108, and re-openable attachment means such as a screw thread 105 for the reservoir 107. In one version, the physical construction of the trap includes the provision that the inlet 20 pipe 109 enters the trap (at 102) at a higher height than the height at which the outlet pipe leaves the trap (at 103) so that, when in use, excess fluid (if any) will tend to flow through the outlet pipe and into a drain rather than re-enter the air-conditioning apparatus and be spread about the building being conditioned. This version is not illustrated. 25 In another version (see the longitudinal section in Fig 4), both the inlet 102 and the outlet 103 of the trap are coaxial; sharing the same axis with each other. The required asymmetry causing a preferred direction of flow is best provided by an overall gentle downward slope (down to the left of Fig 4). Since the trap is usually inserted, by cutting out a short pipe length, into a drainage pipe having, by either good practice or by local 30 ordinances, an approximately 5 degrees downward slope, excess water collected in the 5 trap will first drain into pipe 110. The coaxial inlet and outlet allow the trap to more easily be added to an existing pipe as an in-line accessory since both the inlet and the outlets of the trap are coaxial. The actual trap and seal against transport of noxious vapours is formed around a down 5 feeding internal pipe 112 which reaches down into 106 and below the surface of the reservoir of water, if any. In Fig 4 the water is shown as reaching higher up into this pipe, in the event of a partial vacuum being created within inlet pipe 109 by fan action within the associated air conditioner (not illustrated). The breather pipe 108 emerges from the second chamber 1 10A at the outlet side of the housing and is directed 10 vertically upward. A constricted outlet (10 mm diameter) at 108A moulded into the housing 101 reduces the risk of evaporation of the charge of water from inside the trap. Breather pipe 108 serves as a pressure relief pipe in case fluctuating pressure within the box containing the heat exchanger coils overcomes the head as set within internal pipe 112 and either siphon or blow water out of the traps. With the breather, the trap never 15 operates at a pressure much higher or lower than atmospheric pressure. The inventors prefer that the total height of the invention is minimised for use in restricted spaces, and is about 100 mm, (excluding the breather tube 108). The second major part of the invention comprises a re-attachable, transparent container 107 serving as the reservoir. Fig 3 illustrates a preferred bowl. This comprises a 20 cylindrical water container preferably including fluted finger grips and having a male thread around an open end for attachment to housing 101. A circumferential "0" ring (not shown) laid upon the circumferential ledge 107A under the thread provides secure attachment and also a seal. The preferred O-ring type rubber seal included at the joint has a thickness of about 0.2 mm - 0.3 mm. Lubrication for the O-ring is preferably 25 provided so that compression of the O-ring and retention of the reservoir may be achieved. It is more convenient to include the "0" ring with the or each bowl. The reservoir 107 has an internal cavity 106 for holding water 111 and typically has a capacity for about 225 ml of water. A preferred material from which this bowl may be moulded by, for instance, injection moulding is an optical grade polyamide "Grilamid 30 TR90" (www.emsgrivory.com). The space 106 within the reservoir is open to the 6 second chamber 1 1OA and indirectly, though the internal pipe, to the first chamber 109A. The internal tube, shown in section as 112 in Fig 4, directs the incoming air into the water 111 contained within the reservoir. The submerged tip of tube 112 serves as a seal 5 for attempted air flow in either direction and thereby allows excess water to flow from a drip tray within the air conditioner, through the trap, and into a drain or sewer without either a risk of flooding from the drip tray or of smells which might otherwise be sucked from the drain and distributed around a building with the air-conditioned air flow. Note that when in use, the trap is oriented substantially vertically with the reservoir 107 sited 10 below the housing 101 of the trap, because it is expected that the floor of the container will become covered with a trapping liquid. Of course there will be times, as with all traps, when no liquid is present. VARIATIONS The reservoir 107 may alternatively be made of clear glass, or a clear and inert plastics 15 material, such as an acrylic, or "Mylar®" or other polyethylene terephthalates, or other options including alloys or blends, as are known to one skilled in the art. The reservoir itself may include a biocidal material such as an oil or wax-like material, miscible with the plastic, within the substance of its plastics material from where it is capable of being slowly released into the interior 106, The preferred option is a pellet 20 containing a slowly released form of biocidal material such as hypochlorite ("Milton"@ Sterilising Tablets) that may be inserted either at the time of assembly, or during periodic inspection or maintenance after undoing the reservoir. The inventors note that swimming-pool chlorine tablets tend to react with the reservoir walls. On the other hand, the replaceable reservoir may be a useful public-health detection 25 point for pathogenic organisms such as Legionella spp likely to breed within pools of water associated with air conditioners. If there is a program of routine checks, bowls may be deliberately non-bactericidal and may be removed and sealed with an accompanying lid from time to time and removed for bacteriological growth tests. 7 The serviceman or inspector may be supplied with matching lids (not shown) for sealing used reservoirs, so that he or she can safely carry them away from the site for tested, and cleaning or disposal. General plumbing applications for this trap also exist. For example, the trap can be 5 inserted into drains from drip trays around internal roof tanks and anywhere that condensed, spilled, or slowly leaked water is likely to collect, such as in kitchens and bathrooms in dwellings, and in industrial sites. ADVANTAGES 1. The trap overcomes a risk that the air conditioner drip tray will fill with water 10 and overflow, causing water damage to property such as ceilings, furniture and office equipment located below, or at least facilitates provision of effective and easily maintained drainage means. Pooled water may allow growth of pathogenic organisms such as Legionella spp which have been implicated in a number of deaths arising from breathing droplets of water containing the organisms. 15 2. The trap is functionally more effective than a "U" trap made of pipe sections. For example it includes a controlled amount of asymmetry thanks to the drop pipe 112. The reservoir of the trap holds about 225 ml of liquid; about double the capacity of prior-art traps. 3. The state of the trap is easily inspected, because the presence of a liquid within 20 is easily seen through the transparent walls of the reservoir. As a result the sealing property of the trap is ensured, so that smells arising from a drain are not conducted through the space being air-conditioned. Some prior-art traps had to be disassembled in order to find out whether they contained water. There is no need to cut and rejoin pipes as was the case for prior-art traps. 25 4. The trap is easily cleaned by unscrewing the reservoir, and during cleaning, all contaminants remains accessible and can be disposed of or taken away for testing, such as by screwing on a separate lid and replacing a new or cleaned reservoir on the trap. Maintenance workers need not carry liquid bleach. 5. The total cost of maintenance is substantially reduced. 8 6. Installation time is substantially reduced. There is no need to create a trap from several pipe connectors glued together into a P shape, as was the case for prior-art traps. 7. The low profile of the trap allows installation within a limited height such as that 5 between a false ceiling and a roof structure. A reduced ceiling height has architectural advantages. 8. The amount of liquid within the trap can be checked and maintained, if too low, by easy removal and replacement of the reservoir or by filling down the breather tube. 9. Loss of trap water caused by evaporation is reduced. 10 Finally it will be understood that the scope of this invention as described and/or illustrated herein is not limited to the specified embodiments. Those of skill will appreciate that various modifications, additions, known equivalents, and substitutions are possible without departing from the scope and spirit of the invention as set forth in the following claims. 15 9