inboard ends 658 of the first 650 and second 654 portions of the horizontal rails 182 102 106 110 134 138 142 146 the alignment pins 726 730 are sized shaped and located to fit slidably within the arcurate alignment slots 710 714 each of the pairs of positioning detents 718 722 is spaced apart by a distance slightly greater than a diameter 734 of one of the horizontal rails 182 102 106 110 134 138 142 146 when the first 650 and second 654 portions of the horizontal rails 182 102 106 110 134 138 142 146 are collinear the rails 182 102 106 110 134 138 142 146 will be within the spring housing 702 and when the rails 182 102 106 110 134 138 142 146 are pivoted with respect to one another to fold the rigid frame 14 the detents 718 722 will be urged against the rails 182 102 106 110 134 138 142 146 by the spring resistance of the housing 702 causing the housing 702 to spread apart this resistance serves to maintain the collinear alignment of the rails 182 102 106 110 134 138 142 146 when the rigid frame 14 is erected.