FIG. 1 is a front left bottom perspective view of the pen according to the present design. FIG. 2 is a left side elevation view thereof; FIG. 3 is a right side elevation view thereof; FIG. 4 is a top plan view thereof; FIG. 5 is a bottom plan view thereof; FIG. 6 is a front elevation view thereof; and, FIG. 7 is a rear elevation view thereof.