Shelf

FIG. 1 is a perspective view of a shelf showing our new design; FIG. 2 is a front elevational view thereof; FIG. 3 is a back elevational view thereof; FIG. 4 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof; FIG. 5 is a top plan view thereof; and, FIG. 6 is a bottom plan view thereof. Broken lines in the figures illustrate portions of the shelf that form no part of the claimed design.