The ornamental design for a stool, as shown and described.
FIG. 1 is a perspective bottom side view of a stool showing the seat of the stool positioned above the base portion of the stool;
FIG. 2 shows a front side view of the embodiment shown in FIG. 1. The back, left and right side views are not shown because they have the same appearance as that of FIG. 2;
FIG. 3 is a top view of the design embodiment shown in FIG. 1; and,
FIG. 4 is a bottom view of the design embodiment shown in FIG. 1.