The ornamental design for a sofa, as shown and described.
FIG. 1 is a side and front perspective view of a sofa showing my new
design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view thereof, taken from the left of FIG. 2,
the elevational view opposite that shown in FIG. 3 being the mirror image
thereof;