proposition: Lemma fixed_gal K E x a : (K <= E)%VS -> x \in 'Gal(E / K) -> a \in K -> x a = a.
proof: Proof. by move/gal_kHom=> -> /kAHomP idKx /idKx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype tuple finfun bigop ssralg poly) mathcomp(polydiv finset fingroup morphism quotient perm) mathcomp(action zmodp cyclic matrix mxalgebra vector) mathcomp(falgebra fieldext separable)