The ornamental design for a bag, as shown and described.
FIG. 1 is a front elevational view of a bag, showing my new design, the rear elevational view being a mirror image thereof;
FIG. 2 is a left side elevational view thereof, the right side elevational view being a mirror image thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front perspective view thereof;
FIG. 6 is a front elevational view thereof, showing the bag in a typical configuration for actual use, the rear elevational view being a mirror image thereof;
FIG. 7 is a left side elevational view thereof, the right side elevational view being a mirror image thereof;