Sig.ARITY
Arity of function symbols. 'r represents the result type and 'a, 'b, 'c the arguments.
'r
'a
'b
'c
type 'r ar0
type ('a, 'r) ar1
type ('a, 'b, 'r) ar2
type ('a, 'b, 'c, 'r) ar3