The ornamental design for a hammock, substantially as shown in the attached drawings.
FIG. 1 is a perspective view of the hammock, with a removable netting cover, and having side pockets on each of the two sides of the hammock (only one side is visible in FIG. 1), wherein straps to support the hammock are shown in dashed lines and do not form a part of the hammock design;
FIG. 2 is another perspective view of the hammock shown in FIG. 1, wherein the netting cover is shown removed and rolled up at the foot of the hammock;
FIG. 3 is a left plan view corresponding to FIG. 1, wherein the right plan view is a mirror image of the right plan view and therefore not shown;
FIG. 4 is a back (or head) plan view corresponding to FIG. 1, wherein the pockets on each side can be seen hanging down from the hammock;
FIG. 5 is a front (or foot) plan view corresponding to FIG. 1, wherein the pockets on each side can be seen hanging down from the hammock;
FIG. 6 is a top plan view corresponding to FIG. 1; and,
FIG. 7 is a bottom plan view corresponding to FIG. 1.