An invocation polymorphic ty checks to see if ty has an occurrence
of any type variable. It is equivalent in functionality to
not o null o type_vars, but may be more efficient in some situations,
since it can stop processing once it finds one type variable.
FAILURE
Never fails.
EXAMPLE
- polymorphic (bool --> alpha --> ind);
> val it = true : bool
COMMENTS
polymorphic is also equivalent to exists_tyvar (K true), and no faster.