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