op_arity
Type.op_arity : {Thy:string, Tyop:string} -> int option
Return the arity of a type operator.
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