# Library prosa.util.nat

From mathcomp Require Import ssreflect ssrbool eqtype ssrnat seq fintype bigop div.
Require Export mathcomp.zify.zify.
Require Export prosa.util.tactics.
To aid rewriting steps in larger proofs, we note two simple facts about natural numbers.
Given m ≥ p and n ≥ q, an expression (m + n) - (p + q) can be rewritten as expression (m - p) + (n - q).
Fact subnACA {m n p q} :
p ≤ m → q ≤ n →
(m + n) - (p + q) = (m - p) + (n - q).
Proof. by lia. Qed.
We note that m + p ≤ n implies m ≤ n - p. Note that this fact is similar to ssreflect's lemma leq_subRL; however, this fact avoids the precondition n ≤ p since it is only an implication.
Fact leq_subRL_impl {m n p} : m + n ≤ p → n ≤ p - m.
Proof. by lia. Qed.