The ornamental design for a sofa, as shown and described.
FIG. 1 is a front perspective view of a sofa in accordance with the new design, showing both headrests in an upright position;
FIG. 2 is a front elevation view thereof, showing both headrests in the upright position;
FIG. 3 is a left side elevation view thereof, showing one headrest in a reclined position and the other headrest in the upright position;
FIG. 4 is a right side elevation view thereof, showing one headrest in the reclined position and the other headrest in the upright position; and,
FIG. 5 is a back elevation view thereof, showing one headrest in the reclined position and the other headrest in the upright position.
Broken lines shown on the sofa are understood to represent stitching. Dotted lines shown in FIGS. 3–5 are for illustrative purposes only and form no part of the claimed design.