Returns the set of free variables in a term, in order.
An invocation free_vars_lr ty returns a list representing the set of
type variables occurring in ty. The list will be in order of
variable occurrence when scanning the parse tree of the term from
left to right. This is usually, but need not be, the textual order when
the term is printed.
- free_vars_lr (Term `x /\ y /\ y ==> z`);
> val it = [`x`, `y`, `z`] : term list
free_vars_lr is not efficient for large terms with many free variables.
More strenuous applications should use high performance set implementations
available in the Standard ML Basis Library.
free_vars_lr can be used to build pleasing quantifier prefixes.