I claim the ornamental design for the cap for writing instrument, as shown and described.
FIG. 1 is a front elevational view of the cap for writing instrument showing my design, the back elevational view thereof being a mirror image of the front elevational view thereof;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a perspective view thereof; and,
FIG. 7 is a cross-sectional view thereof, taken along section lines 7—7 in FIG. 1.
The stippling shown on the cap of writing instrument represents a surface texture.