The ornamental design for a cap for writinginstrument, as shown and described.
FIG. 1 is a front elevational view of a cap for writing instrument according to the present invention;
FIG. 2 is a right side elevational view of the cap for writing instrument of FIG. 1, the left side view being a mirror image of the right side view;
FIG. 3 is a rear elevational view of the cap for writing instrument of FIG. 1;
FIG. 4 is a top plan view of the cap for writing instrument of FIG. 1;
FIG. 5 is a bottom plan view of the cap for writing instrument of FIG. 1; and,
FIG. 6 is a front perspective view of the cap for writing instrument of FIG. 1.
The broken lines showing the inside of the cap in FIG. 5 are included for the purpose of illustrating environment and form no part of the claimed design.