fact: Definition step m n (p : {poly R}) := \sum_(l < 2 ^ m) let le := \poly_(i < 2 ^ n) p`_(i + l * 2 ^ n.+1) in let ri := \poly_(i < 2 ^ n) p`_(i + l * 2 ^ n.+1 + 2 ^ n) in merge n le ri * 'X^(l * 2 ^ n.+1).
imports: mathcomp(all_ssreflect all_algebra ssrnum)