FIG. 1 is a top perspective view of a mullion for a window showing my new design;
FIG. 2 is a left side view thereof, the right side view being a mirror image;
FIG. 3 is an enlarged front view thereof, the rear view being a mirror image thereof; and,
FIG. 4 is a top view thereof, the bottom view being a mirror image.
The broken lines illustrate portions of the mullion for a window that form no part of the claimed design.
The mullion for a window is shown with a symbolic break in its length. The appearance of any portion of the mullion for a window between the break lines forms no part of the claimed design.