EXPLANATION STRUCTURE. SAFE-TO-STACK (OBJ1,OBJ2) LIGHTER (OBJ1, OBJ2) WEIGHT (OBJ1, 1) LESS (1, 5) WEIGHT(OBJ2, 5) VOLUME (OBJ1, 1) DENSITY (OBJ1, .1) ISA(OBJ2, ENDTABLE)