The ornamental design for a sofa, as shown and described.
1. Sofa
1.1: Perspective
1.2: Perspective
1.3: Front
1.4: Back
1.5: Left
1.6: Right
1.7: Top
1.8: Bottom
1.9: Enlarged
1.10: Enlarged
1.11: Enlarged
Fig. 1.9is an enlarged view of a portion labeled “9” in fig. 1.1; Fig. 1.10is an enlarged view of a portion labeled “10” in fig. 1.1; Fig. 1.11is an enlarged view of a portion labeled “11” in fig. 1.2; The dash dot dash lines in fig. 1.1, 1.2, 1.9, 1.10, 1.11and the label “9”, “10” in fig. 1.1, label “11” in fig. 1.2are for the purpose of depicting the cut line of the enlarged view and form no part of the claim.