+ * GSTONE(OBJ) = true if OBJ is a gemstone
+ * FOREST(LOC) = true if LOC is part of the forest
+ * OUTSID(LOC) = true if location not in the cave
+ * INSIDE(LOC) = true if location is in the cave or the building at the beginning of the game
+ * INDEEP(LOC) = true if location is in the Hall of Mists or deeper
+ * BUG(X) = report bug and exit
+ */