Checks if a type variable satisfying a given condition exists in a type.
DESCRIPTION
An invocation exists_tyvar P ty searches ty for a type variable
satisfying the predicate P. The value true is returned if the search
is successful; otherwise false is the result.
FAILURE
If P fails when applied to a type variable encountered
in the course of searching ty.
EXAMPLE
- exists_tyvar (equal beta) (alpha --> beta --> bool);
> val it = true : bool
COMMENTS
This function is more efficient, in some cases, than exists P o type_vars.