(fact (num 1))
(fact (num 2))
(fact (num 3))
(fact (num 4))
(fact (num 5))
(fact (num 6))

(fact (add 1 1 2))
(fact (add 1 2 3))
(fact (add 1 3 4))
(fact (add 1 4 5))
(fact (add 1 5 6))

(fact (add 2 1 3))
(fact (add 2 2 4))
(fact (add 2 3 5))
(fact (add 2 4 6))

(fact (add 3 1 4))
(fact (add 3 2 5))
(fact (add 3 3 6))

(fact (add 4 1 5))
(fact (add 4 2 6))
(fact (add 5 1 6))

(fact (mul 2 2 4))
(fact (mul 2 3 6))
(fact (mul 3 2 6))

; (eval E V) if the value of E as a Scheme expression is V.  E may
; consist only of integers 1-6 and expressions (+ E1 E2) and (* E1 E2).
(fact (eval ?n ?n) (num ?n))
(fact (eval (+ ?a ?b) ?v) (add ?a1 ?b1 ?v) (eval ?a ?a1) (eval ?b ?b1))
(fact (eval (* ?a ?b) ?v) (mul ?a1 ?b1 ?v) (eval ?a ?a1) (eval ?b ?b1))

(query (eval (+ ?x ?y) 6))