First and second U-shaped cross-members each include a front portion, a rear portion, and a cross bar connecting the front portion to the rear portion. The front and rear portions of the cross-members are pivotally connected with each other at central portions thereof. Left and right frame side assemblies are pivotally connected to the cross-members and have lateral portions that extend generally parallel the cross bars of the cross-members. The cross-members are mutually movable from a generally X-shaped setup condition in which the cross bars of the cross-members are transversely spaced apart from one another and the lateral portions of the left and right frame side assemblies also are transversely spaced apart from one another, to a generally flat collapsed condition in which the left and right frame side assemblies are nested within an envelope defined by the collapsed cross-members.

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