; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
env2 = extendIdSubst env case_bndr rhs
; return (env2, scrut `Cast` co, case_bndr2) }
; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
env2 = extendIdSubst env case_bndr rhs
; return (env2, scrut `Cast` co, case_bndr2) }