Lines 1 and 2 there don't mean different things. They're both attempting to bind/unify `a` with a value. It's just that in the second case, `a` already has a di

Lines 1 and 2 there don't mean different things. They're both attempting to bind/unify `a` with a value. It's just that in the second case, `a` already has a different value, so the binding fails.