The ornamental design for a cover, as shown and described.
FIG. 1 is a perspective rear view of one embodiment of the inventive cover, which has an inner side thereof provided with a lithophane-type image; and,
FIG. 2 is a perspective top view of the inventive cover.
Activity apparatus and method for generally instantaneously creating lithophane-type pictorial works comprising translucent material within containers having a transparent see-through wall