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nth_rewrite incorrectly unifies universe variables
In the following example, nth_rewrite incorrectly instantiates the universe metavariable of h with 0, but still rewrites with the hypothesis. Note that rw does this correctly.
def foo (h : ∀ {α : Type*} (x : α), id x = x) : id tt = tt :=
begin
nth_rewrite 0 [h], -- error in resulting proof term
-- rw [h], -- works
end