The ornamental design for a stool, as shown and described.
FIG. 1 is a top, front, right side perspective view of a stool showing my new design;
FIG. 2 is a right side elevation view thereof, the left side elevation view being a mirror image of FIG. 2;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The dot-dash broken lines in the figures represent boundaries of the claimed design and form no part thereof. The oblique shade lines in the figures illustrate transparent surfaces.