fact: Definition extension_of r U V := exists2 e : ext_data, r.-tower U (ext_ep e) (ext_pw e) & << U & ext_ep e >>%VS = V.
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)