I FOLDABLE STRIP FOR CONSTRUCTING A PACKAGE Field of the invention THIS INVENTION relates to a strip from which a package such as a pharmaceutical dosage package may be constructed. More specifically the strip is of planar form and may be provided to a pharmacist to enable the construction of a disposable blister package having a blister sheet containing within the cavities of its sealed blisters doses of one or more prescribed medications. State of the art A commercially available and book-like disposable blister package is available which is capable of being easily stored and is of relatively small size so that it can be carried in a patient's pocket or handbag. This package is formed from inner and outer stiff cards provided with parallel fold lines and arranged face-to-face. The cards are joined to one another along a central hinging section and the inner card lies within the boundary of the outer card. The outer card provides the outside of the package and the inner card provides with its opposite end-portions two leaves sub-divided into separable parallel fingers in each of which are two sealed blisters containing respective tablets. The blisters are individually detachable from the leaves when required. The above-described blister package can be produced in bulk in a factory. However it is not suitable for assembly by a pharmacist in a retailing pharmacy. Object of the invention An object of this invention is to provide a sheet from which a pharmacist can easily and quickly assemble a disposable blister package. The invention In accordance with the present invention a strip for supporting a blister sheet to be loaded with prescribed medication doses and from which a disposable blister package 2 can be constructed, comprises a rectangular card divided by at least one hinge line into a pair of substantially identically shaped rectangular panels each having an array of openings for accommodating respective finger-depressible blisters of a blister sheet and which register with one another when the two panels are folded together about the hinge line, one panel of the pair having one side covered by one face of an easily-ruptured foil the other face of which supports an adhesive layer provided with non-adhesive areas and covered by a removable cover slip which, when detached, exposes the adhesive layer and enables the two panels of the pair to be brought together face-to-face so that the adhesive layer secures them together with the non adhesive areas respectively covering the mouths or respective blister cavities of a blister sheet which is supported by the other panel of the pair. Advantages of the invention A card as described above can be stored flat with other similar cards before use by a pharmacist, and can be folded and used when combined with a blister sheet to provide a disposable blister package. Preferred but optional features of the invention Preferable the card provides two pairs of panels each arranged as described above. A thin rectangular area extending across the card may be used to hold the two panels in a spaced relationship when the card is folded. Four parallel hinging lines are then necessary. Conveniently the openings on the two panels are staggered with respect to one another so that when the two pairs of panels are folded together they provide a blister package in which the array of blisters on one pair of panels inter-digitate with respect to the array of blisters on the other pair of panels. The non-adhesive areas may be provided by using an adhesive layer having an appropriately shaped pattern. However other techniques may be used. For example the adhesive layer may be continuous and the cover slip may be slit along lines to 3 define closed zones each constituting a non-adhesive area. Removal of the cover slip by the pharmacist leaves the closed zones in place so that when a pair of panels are folded together the closed zones cover the mouths of respective blister cavities. A further technique is to apply to the adhesive layer at appropriate positions and before the cover slip is applied, a suitably-shaped, thin and easily rupturable, non-adhesive shape. Finally, an adhesive may be used which relies on heat and pressure to seal it to co-planar lands of the blister sheet lying between the blisters. The adhesive layer is otherwise inert so that it will have no deleterious effect on medication doses which may be contained within the sealed blisters. To ensure that the finger pressure necessary to exert on a blister in order to rupture the foil, is not significantly increased by the presence of a non- adhering shape of material covering the mouth of the blister, the material may be fibrous or may be provided with a series of fine slits so that it breaks up easily. In the preferred arrangement of blister sheet each blister communicates with a neighboring blister by a small air duct or channel as is described in patent number 779458. When the blister from which the doses are to be extracted is depressed by finger pressure, some of the air within the cavity of the blister flows into the neighboring blister to dilate it slightly so that there is no build up of pressure in the blister being depressed. Introduction to the drawings The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: In the drawings FIGURE 1 is a top plan view of a rectangular foldable strip for supporting two blister sheets to provide a compact disposable blister package; FIGURE 2 is an under plan view of the strip of figure 1; 4 FIGURE 3 is a diagrammatic and partially exploded view of the sheet as seen in the direction of the arrow "A" in figure 1; FIGURE 4 shows diagrammatically a stage in the assembly of the blister package from the strip shown in figure 2; and, FIGURE 5 is a diagrammatic end view of the blister package stowed within a protective open-ended parallelepiped sleeve. Description of preferred embodiment The strip of figure 1 comprises a stiff card 1 having four spaced parallel fold lines 2 formed by lines of weakness such as perforations and dividing the card into five rectangular panels 3,4,5,6 and 7 each extending between opposite side edges 9 of the card. The panels are arranged two identical pairs 8 and 10 and the central panel 5 provides spine about which the two pairs of panels 8 and 10 can be turned. The panels 3,4 6 and 7 are each formed with an array of oblong openings 11. The openings 11 of each of the panels of a pair register with one another when the panels of a pair are folded together face-to-face as shown in figure 5. Referring back to figures 1 and 3, the panels 3 and 7 are each covered on one side of the card 1 by a respective adhering thin backing foil 12 and 13 which can be easily ruptured. The foils cover the array of openings 11 on the panels 3 and 7 and provide effective seals around the openings 11 by virtue of their adherence to the marginal portions of the card 1 surrounding them. A layer 14 of a pressure-sensitive adhesive is applied to the exposed surface of each of the foils 12 and 13, the layer having a pattern such that it does not cover any of the oblong openings 11 beneath. A thin, flexible cover slip 15 protects the surface of the 5 adhesive layer 14 from damage and may be peeled off to expose the layer 14 when required during the assembling of the blister package 16 shown in figures 4 and 5. Assembly of preferred embodiment The assembly of the blister package is carried out as follows. The card I together with its adhering foils 12 and cover slips 15 is provided flat to a pharmacist. When a blister package is to be assembled, two rectangular blister sheets 17 are arranged on the card I so that their blisters 18 locate in the openings 11 of respective panels 5 and 7 as shown in figures 2 and 3. The mouths of the blister cavities are then uppermost so that they can be loaded with medication doses prescribed by a doctor. After the blisters have been loaded with their doses, the protective cover slips 15 are peeled off the adhesive layers 14 and the panels 3 and 7 are folded about the hinging lines 2 to bring the adhesive layers 14 into contact with the coplanar lands of the blister sheets 17. These lands extend between the mouths of the blisters 18. Pressure is then applied. The nature of the adhesive of the layer 14 is such that when it is cold pressed against the lands, it secures the panels 3 and 7 to the lands and provides a hermetic seal around the mouths of each of the blisters 18. Referring to figure 4, the blister package is finally assembled by folding each of the pairs of panels 8 and 10 towards one another as shown by the arrows, so that their blisters lie within the confines of the package as shown in figure 5. Legend on each of the two pairs and of panels 8 and 10 denotes the days on which the contents of the blisters are to be administered and the time of the day at which such administration is to take place. An open-ended cardboard sleeve 20, as shown in figure 5, is used to protect the blister package when being carried in a handbag or pocket. The package can be slid 6 out of the sleeve 20 when required for use and then replaced in the sleeve afterwards. When all of the blisters have been opened the package can be thrown away. Variations and/or modifications of preferred embodiment The invention is applicable to blister package having a single blister sheet. It is also susceptible to many modifications. For example, the non-adherent areas of the layer 14 may be provided by attaching appropriate shapes of a non-adherent material over the relevant areas of the adhesive layer before the cover slip is applied. This enables a continuous adhesive layer without a special pattern to be used. In another modification the cover slip is provided with slits defining closed zones each having the same outline as the mouth of a blister it is intended to cover when the two panels of a pair are folded together. With such an arrangement the peeling-off of the cover slip leaves the closed zones in respective positions at which they close the mouths of the blister cavities. The adhesive layer between the blister cavities then provides a seal around each of the blisters. In a further modification the spinal panel is replaced by a single fold line and the blister package is folded so that the two pairs of panels lie against one another with the blisters protruding outwardly rather than inwardly. In yet another modification more than four parallel fold lines are provided so that the number of panels available for sealed blister sheets is increased from two to three or more. Each of the blister sheets is similarly sealed in the way described above. Finally the card can be arranged so that the array of openings on one pair of panels are offset with respect to the array of openings on the other pair of panels. The openings can then be spaced from one another to enable the blisters on one blister sheet to inter-digitate with the blisters of the other blister sheet. Such an arrangement reduces the thickness of a package containing two blister sheets at the expense of slightly enlarging its length.