We claim, the ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of a bag, showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The evenly dashed broken lines on claimed surfaces depict claimed stitching. All other broken lines depict portions of the article and form no part of the claim.