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
1.8: Rear perspective
The bag comprises a body with a rectangular front wall and a rectangular rear wall, a flattened rectangular bottom, two side pleats and a front flap that covers the upper part of the front wall; the bottom end of the flap has a reduced width and is 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; a handle is attached to the top of the body; the front wall of the body has a quadrangular opening where a pocket provided with a zipper is attached; the rear wall has a vertical opening for accessing an inner pocket.