Variable capture when substituting in Rebind patterns

[lambdageek]

Migrated from unbound issue tracker - issue 25

Should be able to detect & fail when capture occurs while substituting in patterns. For example,

subst (Rebind x y) (Var x)

will produce an invalid pattern. The variable x will still be a free variable, even though the pattern binds > variables of that name. This behavior should be caught and an appropriate error message given.

May also be worth having a Maybe version of subst.

[lambdageek]

First step is to create a real test case.