The ornamental design for a sofa, as shown and described.
FIG. 1 is a front left perspective view of a first embodiment of a sofa showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof, the right side view being a mirror image;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a front left perspective view of a second embodiment of a sofa showing our new design;
FIG. 7 is a front elevation view thereof;
FIG. 8 is a left side elevation view thereof, the right side view being a mirror image;
FIG. 9 is a rear elevation view thereof; and,
FIG. 10 is a top plan view thereof.
FIGS. 6, 7, 9, and 10 include a symbolic break shown in broken lines. The broken lines as well as those portions of the article between the break lines form no part of the claimed design.