proposition: Lemma ack'_reification_helper: exists res, interp_term nil res = existT _ (tpArr tpNat (tpArr tpNat (tpArr tpNat tpNat))) ack'.
proof: Proof. eexists. unfold ack'. repeat t. all: reflexivity. Defined.
imports: Arith Lia List FunctionalExtensionality