The ornamental design for a bag, as shown and described.
1. Bag
1.1: Perspective
1.2: Front
1.3: Back
1.4: Right
1.5: Left
1.6: Top
1.7: Bottom
The body includes a substantially rectangular front wall and a substantially rectangular rear wall, a flattened rectangular bottom, two side pleats and a front flap with three scallops, closed by two straps that extend from the rear wall of the body through openings in the side pleats and the front wall, the ends of said straps being coupled by means of a clasp; two handles are attached to the front wall and to the rear wall of the body; the ends of the front handle extend through slots between the scallops of the flap; the front wall of the body has a quadrangular opening where a pocket provided with a zipper is attached; the rear wall has a horizontal opening for accessing an inner pocket.