The ornamental design for a shelf, as shown and described.
FIG. 1 is the front/right-side/top perspective view of a shelf showing our new design;
FIG. 2 is the front-side elevation view thereof;
FIG. 3 is the rear-side elevation view thereof;
FIG. 4 is the left-side elevation view thereof;
FIG. 5 is the right-side elevation view thereof;
FIG. 6 is the top-side elevation view thereof;
FIG. 7 is the bottom-side elevation view thereof;
FIG. 8 shows the shelf in a usage position; and,
FIG. 9 shows the shelf installed in an unclaimed environment.
The inventors contemplate that other embodiments of their design may include any part, portion, element or combination of elements of the claimed design.
The broken lines in the drawing show unclaimed environment only and form no part of the claimed design.