The ornamental design for a foldablestool, as shown and described.
FIG. 1 is a perspective view of a foldable stool showing my new design disposed, by way of example only, with a pair of arms in a first position;
FIG. 2 is a top view of the foldable stool of FIG. 1 with the pair of arms in the first position;
FIG. 3 is a perspective view of the foldable stool of FIG. 1, with the seat and handle in a lowered position and the pair of arms in a folded second position;
FIG. 4 is a top view of the foldable stool of FIG. 1 with the seat in a lowered position and the pair of arms in a folded second position; and,
FIG. 5 is a side view of the foldable stool of FIG. 1 with the pair of arms moving between the first position and the second position.