Table 2. The SAFE-To-STACK generalization problem after Borgida et al. (1985) Given Goal Concept: Pairs of objects <x, y> such that SAFE-TO-STACK (x, y), where SAFE-TO-STACK(x, y)* NOT(FRAGILE (y))V LIGHTER (x, y) Training Example. ON (OBJl, OBJ2) ISA(OBJ1, BOX) ISA(OBJ2, ENDTABLE) COLOR(OBJl, RED) COLOR(OBJ2, BLUE) VOLUME (OBJl, 1) DENSITY(OBJ1, 1) · Domain Theory. VOLUME (pl, v1)A DENSITY (pl, di)- WEIGHT(pl, v1.d1) WEIGHT(pl, wl)A WEIGHT (p2, w2)A LESS (wl, w2)- LIGHTER(pl, p2) ISA(pl, ENDTABLE)- WEIGHT(pl, 5)(default) LESS (.1, 5) G berationality Criterion The concept definition must be expressed in terms of the predicates used O describe examples(e. g, VOLUME, COLOR, DENSITY) or other selected, easily evaluated predicates from the domain theory (e. g, LESS). Determine A generalization of training example that is a sufficient concept definition for the goal concept and that satisfies the operationality criterion