Shelf

FIG. 1 is a top perspective view of a shelf in accordance with the present design; FIG. 2 is a front elevation view thereof; FIG. 3 is a rear elevation view thereof; FIG. 4 is a right side elevation view thereof; FIG. 5 is a left side elevation view thereof; FIG. 6 is a top plan view thereof; FIG. 7 is a bottom plan view thereof; FIG. 8 is a bottom perspective view thereof; and, FIG. 9 is an alternate top perspective view thereof. The broken lines shown in the drawings illustrate portions of the shelf that form no part of the claimed design.