The ornamental design for a bag, as shown and described.
FIG. 1 is a perspective view of a bag showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a perspective view of the bag where the bag is in a close state.
The thick short even-spaced broken lines in the drawings depict stitching that form no part of the claimed design. The thin long even-spaced broken lines in the drawings depict portions of the bag that form no part of the claimed design.