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;
FIG. 9 is a perspective view of the bag where the bag is folded into an alternate configuration of use; and,
FIG. 10 is another perspective view of the configuration of FIG. 9.
The evenly dashed broken lines in the drawings depict stitching of the bag and form no part of the claimed design. The dash-dot-dash broken lines in FIG. 7 depict portions of the article and form no part of the claimed design.