fact: Fixpoint dp_eval_pfree φ p := match p with | dp_nat_pfree n => n | dp_var_pfree v => φ v | dp_comp_pfree do_add_pfree p q => dp_eval_pfree φ p + dp_eval_pfree φ q | dp_comp_pfree do_mul_pfree p q => dp_eval_pfree φ p * dp_eval_pfree φ q end.