+substIdBndr :: SimplEnv -> InBndr -> (SimplEnv, OutBndr)
+-- Might be a coercion variable
+substIdBndr env bndr
+ | isCoVar bndr = substCoVarBndr env bndr
+ | otherwise = substNonCoVarIdBndr env bndr
+
+---------------
+substNonCoVarIdBndr
+ :: SimplEnv
+ -> InBndr -- Env and binder to transform
+ -> (SimplEnv, OutBndr)