I claim the ornamental design for a fan, as shown and described.
FIG. 1 is a perspective view of a fan showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a side view from the side opposite that of FIG. 4;
FIG. 6 is a plan view thereof from above; and,
FIG. 7 is a plan view thereof from below.
The broken line showing of structural elements is included for the purpose of illustrating portions of the article and forms no part of the claimed design.