The ornamental design for a stringretainer, as shown and described.
FIG. 1 is a top view of a string retainer showing my new design.
FIG. 2 is a bottom view thereof.
FIG. 3 is a left side view thereof. The right side view thereof is identical to FIG. 3.
FIG. 4 is an end view thereof. The opposite end view thereof is identical to FIG. 4.
FIG. 5 is a top perspective view thereof.
FIG. 6 is a top view of an alternative embodiment string retainer; and,
FIG. 7 is a bottom view of the alternative embodiment string retainer. The side and end views of the alternative embodiment are identical to FIGS. 3 and 4, respectively.
The broken lines in FIGS. 6 and 7 represent the portions of the string retainer, which form no part of the claimed design.