The ornamental design for a stool, as shown and described.
FIG. 1 is a rear elevational view of a stool, in accordance with my new design;
FIG. 2 is a bottom plan view of the stool of FIG. 1, the broken line showing of four small rectangular areas being for the purpose of illustrating recesses in the surface shown, the recesses forming no part of the claimed design;
FIG. 3 is a front elevational view of the stool of FIG. 1;
FIG. 4 is a left side elevational view of the stool of FIG. 1;
FIG. 5 is a right side elevational view of the stool of FIG. 1;
FIG. 6 is a top plan view of the stool of FIG. 1, the broken lines on either side of each roller being for the purpose of illustrating axles which form no part of the claimed design; and,
FIG. 7 is a top, front perspective view of the stool of FIG. 1.