I claim the ornamental design for a hammock, as shown and described.
FIG. 1 is a perspective view of a hammock, showing a first embodiment of my new design;
FIG. 2 is a top plan view of FIG. 1;
FIG. 3 is a bottom plan view of FIG. 1;
FIG. 4 is a front elevational view of FIG. 1, the rear elevational view being a mirror image thereof;
FIG. 5 is a left-side elevational view of FIG. 1, the right-side elevational view being a mirror image thereof;
FIG. 6 is a perspective view of a second embodiment thereof;
FIG. 7 is a top plan view of FIG. 6; and,
FIG. 8 is a bottom plan view of FIG. 6.
The broken lines in the figures represent stitching, and are part of the claimed design. It is understood that the left-side, right-side, front and rear elevational views of both embodiments are identical.