+ new_b = uniqAway in_scope bR
+
+-- Note [rnBndrLR]
+-- ~~~~~~~~~~~~~~~
+-- Notice that in rnBndrL, rnBndrR, we extend envR, envL respectively
+-- with a binding [new_b -> new_b], where new_b is the new binder.
+-- This is important when doing eta expansion; e.g. matching (\x.M) ~ N
+-- In effect we switch to (\x'.M) ~ (\x'.N x'), where x' is new_b
+-- So we must add x' to the env of both L and R. (x' is fresh, so it
+-- can't capture anything in N.)
+--
+-- If we don't do this, we can get silly matches like
+-- forall a. \y.a ~ v
+-- succeeding with [x -> v y], which is bogus of course