reserve x,y for set;

theorem
  for F being Field, a,b being Element of NonZero F holds
  omf(F).(a,b) is Element of NonZero F & revf(F).a is Element of NonZero F
proof
  let F be Field, a,b be Element of NonZero F;
  [a,b] in [:NonZero F,NonZero F:];
  hence thesis by REALSET1:def 4;
end;
