Non-fixed roofs; Roofs with movable panels {, e.g. rotary sunroofs} of non-sliding type, i.e. movable or removable roofs or panels, e.g. let-down tops or roofs capable of being easily detached or of assuming a collapsed or inoperative position foldable; Tensioning mechanisms therefor, e.g. struts with a plurality of {rigid} plate-like elements {or rigid non plate-like elements, e.g. with non-slidable, but pivotable or foldable movement}