(type A (enum
  (A1 (x B) (y B))))
(type B (enum
  (B1 (x u32))
  (B2 (x u32))))

(decl A2B (A) B)

(rule 1
  (A2B (A.A1 _ (B.B1 x)))
  (B.B1 x))

(rule 0
  (A2B (A.A1 (B.B1 x) _))
  (B.B1 x))

(rule 0
  (A2B (A.A1 (B.B2 x) _))
  (B.B1 x))

(rule -1
  (A2B (A.A1 _ _))
  (B.B1 42))