The ornamental design for a stool, as shown and described.
FIG. 1 is a perspective view of a stool according to the present invention.
FIG. 2 is a front elevation or a rear elevation view of the stool illustrated in FIG. 1.
FIG. 3 is a side elevation view of the stool illustrated in FIG. 1.
FIG. 4 is a top plan view of the stool illustrated in FIG. 1; and,
FIG. 5 is a bottom plan view of the stool illustrated in FIG. 1.
The broken lines in FIGS. 1, 2, 3 and 5 illustrate environmental features, specifically feet on a bottom portion of the legs of the stool, fasteners used to secure a portion of the stool, and a shelf positioned on a medial portion of the stool, all of which form no part of the claimed design.