The ornamental design for a “case for a writinginstrument,” as shown and described.
FIG. 1 is a perspective view of a case for a writing instrument showing a new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a rear view thereof.
The broken lines are shown for the purpose of illustrating environmental subject matter and parts of the case for a writing instrument that form no part of the claimed design.