The ornamental design for a sofa, as shown and described.
FIG. 1 is a front elevation view of a sofa, showing a back of the sofa in a reclined position, in accordance with the new design;
FIG. 2 is a top elevation view thereof;
FIG. 3 is a front elevation view thereof, showing the back of the sofa in an upright position;
FIG. 4 is a back elevation view thereof, showing the back of the sofa in the upright position;
FIG. 5 is a side perspective view thereof, showing the back of the sofa in the reclined position, an opposite side perspective view being a mirror image thereof; and,
FIG. 6 is a side perspective view thereof, showing the back of the sofa in the upright position, an opposite side perspective view being a mirror image thereof.
The broken lines shown on the sofa are understood to represent stitching.