R TABLE SCREEN ASSEMBLY Field of the Invention This invention relates to a retractable screen assembly for a closure apparatus that has an opening, such as in a door frame or window frame. 5 Backgrounid of the Invention Existing retractable screens that are used to cover an opening, and thereby prevent pests and/or insects from entering an interior area, are typically used with doors that open out onto an outside area. When the door or doors are open the screen is pulled out from a spool of a cassette on which the screen is wound and the screen is placed in a position to 10 cover the open area. Existing screens generally move in tracks separate to tracks in which associated doors or windows move. As a result, the screen does not adequately cover the opening as pests and insects can still gain entry to an interior space through a cavity that exists between the door (or window) and the screen. 15 The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art in Australia. Summary of the Invention According to an aspect of the invention, there is provided a retractable screen 20 assembly associated with a closure apparatus, the closure apparatus having a frame and at least one closure element fitted to or associated with the frame, the screen assembly including: a screen adapted to extend from and retract to a spool on which the screen is stored; retention means for retaining the screen against a member that is not part of the frame; such that the screen is adapted to substantially cover an opening created by the at least one closure element being in an open condition. 5 According to an embodiment the closure apparatus may include at least two closure elements fitted to or associated with the frame, According to a further embodiment the member is one of the at least two closure elements or the closure element of the closure apparatus in a closed condition, such that the screen positively engages with the closed closure element. 10 According to another embodiment the member is a further screen adapted to extend from and retract to a further spool on which the further screen is stored, the screen and further screen adapted for positive engagement through the retention means to substantially cover the opening. Preferably the retention means is formed on or adjacent to a portion of the leading 15 edge/side of the screen and adapted to positively engage the screen to the member. Preferably the retention means is formed on or adjacent to a portion of an edge/side of the member, and adapted to positively engage the member to the screen. Preferably the retention means is formed on each of the screen and the member to positively engage the screen and member to substantially cover the opening. 20 The screen preferably moves within a top screen track and a bottom screen track, such that the screen is kept within the plane of the member when the member is in a closed condition. The width of the screen tracks is preferably less than the width of the member when the member is a closure element. 2 Preferably the closure elements substantially cover the bottom screen track when the closure elements are in a closed condition. Preferably the retention means extends up to half the width of the closure element in the closed condition. 5 When the screen is in a fully retracted position, the screen preferably does not impede the closure elements when each of the closure elements are in a closed condition. The retractable screen assembly may further include a cassette housing containing the spool and be located adjacent a side of the frame. The retractable screen assembly may further include a handle on the screen 10 adjacent a leading edge of the screen. Brief Description of the Drawings Preferred embodiments of the invention will hereinafter be described, by way of example only, with reference to the drawings in which: Figure IA is a front view of a closure apparatus, including a door frame with a 15 cassette housing attached at one vertical side of the frame from which a retractable screen can be extended; Figure lB is a plan sectional view taken on line B-B of Figure lA showing a door in a closed position and with a screen fully retracted in the cassette housing; Figure 1 C is a side sectional view taken along line A-A of Figure IA showing a 20 door extending between a top track and a bottom track associated with the door frame; Figure 2A is a plan view along line B-B of Figure IA showing a pair of doors in a door frame; Figure 2B is a view similar to Figure 2A except with one of the doors in a fully open position and a retractable screen assembly latched or affixed to the other door 25 shown in a closed position; 3 Figure 3A is a sectional view similar to Figure 2A except it shows a pair of bi-fold doors and a further door in a door frame all shown in the closed position; Figure 3B is a view similar to Figure 3A but showing the pair of bi-fold doors in the process of being opened; 5 Figure 3C is a view similar to Figure 3B but showing the pair of bi-fold doors in a fully open position with the remaining door in a closed position and having attached thereto a retractable screen; Figure 4 is a perspective view from above of the position of the various doors and screen shown in Figure 3C; 10 Figure 5A is a plan view of a pair of open bi-fold doors and a pair of fully closed bi-fold doors to which is attached a retractable screen; Figure 5B is a plan view showing a single door open and two pairs of bi-fold doors in a fully closed position with a screen assembly positively attached to one of the pairs of bi-fold doors covering an opening; and 15 Figure 5C is a plan view showing a pair of screen assemblies that are retractable to respective spools at the sides of a door frame, the screen assemblies covering an opening where all sets of doors are in a fully open position. Detailed Description of the Preferred Embodiments With reference to Figure 1A there is shown a closure apparatus having a frame 20 assembly (2) and closure elements (2), such as a door or window or a series of doors or windows, that includes a top portion (4), a bottom portion (6), and a pair of side portions (8 and 9). Located adjacent side portion (8) is a cassette assembly or housing (10) which is used to house a screen on a spool, the screen typically formed of mesh material, and which is able to be retracted and stored on the spool of the cassette assembly (10). A 4 reveal (12) that forms part of frame assembly (2) is located adjacent the cassette assembly (10). The embodiments described herein are specifically described in relation to a door frame or a frame containing a series of doors. It is equally applicable to any other 5 closure assembly, such as a window or a series of windows in a frame. Figure lB shows in detail part of side portion (8) of frame assembly (2) in section. The cassette assembly (10) includes a reel or spool (14) on which is wound screen (16), generally made of a mesh material, and being adapted to slide within a top screen track (27) (Fig. IC) and a bottom screen track (28). The cassette assembly (10) is 10 hidden behind the frame (2) which comprises side portion (8) and reveal (12) and a further reveal (18). A set back (20) is made to be covered with plaster flush with reveal (12). A closure element, in the form of a first door (22), which is shown in a closed position, is adapted for movement within a bottom door track (24) and has a corresponding door top hanger track (26). Bottom screen track (28) is adjacent the 15 bottom door track (24) and forms part of the overall bottom track (30). It is seen that first door (22) substantially covers bottom screen track (28) when the first door (22) is in the closed position. Similarly, with reference to Figure IC, overall top track (32) includes the top screen track (27) and door hanger track (26) which provides space for appropriate hangers to be attached to the top part of door (22). 20 Referring to Figure 2A there is shown a plan sectional view of two closure elements, namely first door (22) and second door (36), both in a closed position. Screen (16) has a handle (34) attached at one side thereof which can be incorporated as a full length handle from the top of the screen (16) to the bottom of the screen (16) or otherwise as a handle that is fitted to a side portion of the screen (16). 5 In Figure 2B there is shown first door (22) in a open position and second door (36) in a closed position, with a retractable screen assembly shown attached to the second door 36. Positioned on side (38) of second door (36) is a retention means (40) in the form of a magnetic strip that extends the entire length of side (38) on one half of the side 5 (38). The width of the retention means (40) can be of a dimension extending up to substantially half of the width of side (38). Other retention means may be used and positioned at various places along side (38). Such retention means may be separate magnetic strips or hook and loop material, for example, placed on the open side of screen (16) or full length handle (34), and the side (38) of second door (36). The retention 10 means (40) is used to hold the screen (16) in place across the opening via handle (34) of the screen (16) or the leading edge/side of screen (16). The screen (16) is extended from the spool (14) of cassette assembly (10) in order to cover the opening that has been created by first door (22) being in an open position. Thus a user grabs handle (34) to pull screen (16) across the opening via the top and bottom screen tracks (27, 28) until screen 15 (16) comes into contact with the retention means (40). It is to be noted that the screen (16) and bottom screen track (28) remain within the width of door (22) as door (22) would be seen in a closed position. In this manner it allows the handle (34) and/or the strip comprising the open/exposed side of the screen (16) to be attached to the retention means (40) in order to cover the opening created by door (22). Thus, the retention means 20 (40) retains the screen (16) against a member, in this case second door (36), that is not part of frame assembly (2). In the retracted position, as shown in Figure 2A, the handle (34) is held in a position such that it is clear of door (22) in the closed position and is prevented from further retractive movement by blocks (42, 44) of the side portion (8). Although the embodiment of Figures 2A and 2B has been described with 25 reference to two closure elements, being first door (22) and second door (36), another 6 embodiment uses a single closure element, such as a door or window. The single closure element may slide/move within the frame between open and closed conditions, for example, in a door track or window track or alternatively against the framer. The closure element may be recessed in a cavity, for example, a wall, or be positioned adjacent a wall 5 or other structure when in a fully or partially open condition. The screen (16) can then be retained against the single closure element by the retention means (40) to close the opening created by the closure element being in an open condition. With reference to Figure 3A there is shown a further embodiment in which a pair of doors (50, 52) constitute a bi-fold door arrangement and a further door (54) which are io all shown in a closed configuration. Each of doors (50, 52) are joined by a series of hinges (56) whilst door (52) has attached at least one roller (58) that rolls within the door hanger track (26). Doors (50, 52) are shown partially open in Figure 3B with door (52) moving to a position where it is folded against door (50) as shown in a fully open position in Figure 15 3C. Door (54) remains in a fully closed position. In Figure 3C, after the doors (50, 52) have been fully folded, the open space thus created can be closed by a user gripping handle (34) of screen 16 and pulling it away from spool (14) to then attach it to the retention means (40) affixed to door (54). Again, the retention means (40) extends along one half of side (41) of door (54). The screen (16) in this position fully covers the 20 opening so that pests and insects cannot enter an interior space. Although one pair of bi fold doors is shown, the retractable screen assembly can be extended to cover a whole series of bi-fold doors that fit within a door frame. Similarly the screen (16) can be adapted to fit across any open space created by the opening of a window or a set of bi fold windows. In order to retract the screen (16) back to its spool (14) it is a matter of 25 disengaging the handle (34) and/or the open side of screen (16) from the retention means 7 (40) until the handle (34) comes to rest against blocks (42, 44) to a fully retracted position. With reference to Figure 4 there is shown a perspective view from above of the closure apparatus and screen assembly shown in Figure 3C, whereby doors (50, 52) are 5 fully open and folded against one another. Door (54) is shown in a closed position to which is attached screen (16) which is shown to cover the open space created by the opening of doors (50, 52). Shown in Figure 5A is a pair of bi-fold doors (66, 67) in a fully open position and a further pair of bi-fold doors (68, 69) shown in a fully closed position. The screen 10 assembly including screen (64) is affixed to a retention means on the side (77) of door (68), which screen (64) is extended from spool (61) in cassette housing (60)- At an opposite side of the door frame is a further cassette housing (62) with spool (63) from which a further screen (65) is in a fully retracted position. Thus screen (64) covers the opening created by the opening of bi-fold door (66, 67). 15 With reference to Figure 5B there is shown a single door (70) in a fully open position with a first pair of bi-fold doors (71, 72) and a second pair of bi-fold doors (73, 74) each shown in a fully closed position. Screen (64) is attached to retention means, particularly in the form of a magnetic strip or strips, attached to side (78) of door (71). Finally in Figure 5C there is shown screens (64, 65) meeting at approximately in 20 the middle of the open area created by all of the doors (70, 71, 72, 73, 74) being in a fully open position. Screen assembly (65) is extendible from spool (63) whilst screen (64) is extendible from spool (61). In this situation either the leading vertical edge of screen (64) or the leading vertical edge of screen (65) will have fitted thereto retention means, or both leading edges may in combination have a retention means fitted, in order to 25 maintain each screen (64, 65) in a positive engagement to close the opening. Thus, the 8 Membe, inthis, case screen (6 5), which retention means retains screen (64) against a member, in tds close elements (70, does not form part of frame assembly (2) which in turn surround 71, 72, 73, 74). that various alteratiOls, modifications and/or additions may It is to be ersto possible and preferred embodiment(s) of the invention as be nad tothe features of the pss herein described without departing from the spirit and scope of the inventifl 9