reserve x,y for set;

theorem
  for F being Field, x,y being Element of F holds x+y = 0.F implies
  y = (comp F).x
proof
  let F be Field, x,y be Element of F;
  assume x+y = 0.F;
  then y = -x by RLVECT_1:6;
  hence thesis by VECTSP_1:def 13;
end;
