fact: Definition area a b := (a.2)%:Z * (b.1)%:Z - (a.1)%:Z * (b.2)%:Z.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype order bigop ssralg) mathcomp(div ssrnum ssrint rat prime path binomial) mathcomp(tuple finset) gaia(fibm)