Mask

FIG. 1 is a front view of a mask showing my new design; FIG. 2 is a rear view thereof; FIG. 3 is a left side view thereof; FIG. 4 is a right side view thereof; FIG. 5 is a top view thereof; FIG. 6 is a bottom view thereof; FIG. 7 is a front, top and right perspective view thereof; and, FIG. 8 is a rear, bottom and right perspective view thereof. The broken lines depict portions of the mask that form no part of the claimed design.