Build.Arity
type nonrec 'r ar0 = (ar0, 'r) term
type nonrec ('a, 'r) ar1 = ('a ar1, 'r) term
type nonrec ('a, 'b, 'r) ar2 = (('a, 'b) ar2, 'r) term
type nonrec ('a, 'b, 'c, 'r) ar3 = unit