The ornamental design for a fan, as shown and described.
FIG. 1 is a bottom and front perspective view of my new and ornamental fan.
FIG. 2 is a top and back perspective view.
FIG. 3 is a left side view.
FIG. 4 is a front view.
FIG. 5 is a right side view.
FIG. 6 is a back view.
FIG. 7 is a bottom view; and,
FIG. 8 is a top view.
The broken line showing of side portions and rear and bottom apertures is included for the purpose of illustrating environmental structure and forms no part of the claimed design.