reserve x,y for set;

theorem
  for F being Field, x being Element of F holds x = (comp F).((comp F).x )
proof
  let F be Field, x be Element of F;
  thus x = --x
    .= (comp F).-x by VECTSP_1:def 13
    .= (comp F).((comp F).x) by VECTSP_1:def 13;
end;
