Shelf

FIG. 1 is a front perspective view of a shelf shown in use, showing our new design; FIG. 2 is a rear perspective view of a shelf in use thereof; FIG. 3 is another front perspective view thereof; FIG. 4 is another rear perspective view thereof; FIG. 5 is a front view thereof; FIG. 6 is a first side view thereof; FIG. 7 is a rear view thereof; FIG. 8 is a second side view thereof; FIG. 9 is a top view thereof; FIG. 10 is a bottom view thereof; and, FIG. 11 is another rear perspective view thereof. The broken lines in the figures illustrate environment and form no part of the claimed design.