The ornamental design for a pen, as shown and described.
FIG. 1 is a front elevational view of a pen showing my new design;
FIG. 2 is a left side elevational view with a portion of the writing tip
being broken away and omitted for convenience of illustration, the
opposite side being a mirror image thereof;
FIG. 3 is a rear elevational view of FIG. 1;
FIG. 4 is a top plan view of the upper end of the pen taken along line 4--4
of FIG. 1; and
FIG. 5 is a bottom plan view of the bottom end of the pen taken along line
5--5 of FIG. 1.