An invocation op_arity{Tyop,Thy} returns NONE if the given record
does not identify a type operator in the current type signature.
Otherwise, it returns SOME n, where n identifies the number
of arguments the specified type operator takes.
FAILURE
Never fails.
EXAMPLE
- op_arity{Tyop="fun", Thy="min"};
> val it = SOME 2 : int option
- op_arity{Tyop="foo", Thy="min"};
> val it = NONE : int option