The ornamental design for a sofa, as shown and described.
FIG. 1 is an upper perspective view of the sofa showing my new design in an upright position and with the pillows in a first position;
FIG. 2 is a front elevation view thereof, with the pillows shown in an alternate position;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a side elevation view thereof, an opposite side view being a mirror image thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a front perspective view thereof, showing the sofa in a fully reclined position and the pillows shown in yet another alternate position.
The broken lines shown on the sofa are understood to represent stitching. The dotted line feet elements shown in the drawings are for illustrative purposes only and form no part of the claimed design.