The ornamental design for a fan, as shown and described.
FIG. 1 is a perspective view of the fan of our new design in a first configuration;
FIG. 2 is an alternate perspective view of the fan of FIG. 1;
FIG. 3 is a front view of the fan of FIG. 1;
FIG. 4 is a rear view of the fan of FIG. 1;
FIG. 5 is a right side view of the fan of FIG. 1;
FIG. 6 is a left side view of the fan of FIG. 1;
FIG. 7 is a top plan view of the fan of FIG. 1;
FIG. 8 is a bottom plan view of the fan of FIG. 1;
FIG. 9 is a perspective view of the fan of our new design in a second configuration;
FIG. 10 is an alternate perspective view of the fan of FIG. 9; and,
FIG. 11 is a rear view of the fan of FIG. 9.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment; the broken lines form no part of the claimed design.