The ornamental design for a set of writinginstruments, as shown and described.
FIG. 1 is a front perspective view of an ornamental design for a set of writing instruments according to the present invention;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a front side elevational view thereof;
FIG. 5 is a rear side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view.
The broken line showing of FIGS. 1-7 is for the purpose of illustrating environmental subject matter and forms no part of the claimed design. The claim is directed to the collective appearance of the set of writing instruments shown in solid lines.