---------
rhs_fvs :: (Id, CoreExpr) -> FV
rhs_fvs (bndr, rhs) = expr_fvs rhs `unionFV`
                      bndrRuleAndUnfoldingFVs bndr
rhs_fvs :: (Id, CoreExpr) -> FV
rhs_fvs (bndr, rhs) = expr_fvs rhs `unionFV`
                      bndrRuleAndUnfoldingFVs bndr
rhs_fvs (bndr, rhs) = expr_fvs rhs `unionFV`
                      bndrRuleAndUnfoldingFVs bndr