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: Front view of the bag open
The bag comprises a body substantially in the form of a horizontal cylinder, a shoulder strap, and a handle in the form of a horse bit comprising two elongate metal elements that are interconnected by a link.