GOAL CONCEPT SAFE-TO-STACK(x, y) R1: SAFE- TO- STACK(p1, p2) lx/p1, y/p2) LIGHTER (p1, D2) LIGHTER (x, y) R2 LIGHTER(p1, p2) x/pl, y/p2) WEIGHT (p1, w1) LESS (w1, w2) WEIGHT (p2, w2) WEIGHT(x, w1) LESS (wf, w2) WEIGHT (y, w2) R3 WEIGHT(p1,v1’d1 D4: WEIGHT (p2, 5) x/p1,v1d1/w11 ly/p2,5/w2 VOLUME(p1, v1) DENSITY (p1, d1) ISA(p2, ENDTABLE VOLUME (x, v1) DENSITY (x, d1) LESS (v1'd1, 5) /SA(, ENDTABLE