proposition: Lemma rext_subspace r E F : r.-ext E F -> (E <= F)%VS.
proof: Proof. by case=> [[/= n e pw] _ <-]; apply: subv_adjoin_seq. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_fingroup all_algebra) mathcomp(all_solvable all_field polyrcf) Abel(various classic_ext map_gal algR) Abel(char0 cyclotomic_ext real_closed_ext)