theorem Th81:
  not M |= x 'in' x & M |= 'not' x 'in' x
proof
  set v = the Function of VAR,M;
  not M,v |= x 'in' x by Th80;
  hence not M |= x 'in' x;
  let v;
  not M,v |= x 'in' x by Th80;
  hence thesis by ZF_MODEL:14;
end;
