
theorem Th20:
  for R being non empty RelStr, N being Subset of R, y being Element of R\~
  st y is_minimal_wrt N, the InternalRel of (R\~)
  holds min-classes N is non empty
proof
  let R be non empty RelStr, N be Subset of R, y be Element of R\~ such that
A1: y is_minimal_wrt N, the InternalRel of (R\~);
  ex x being set st ( x = Class(EqRel R,y) /\ N);
  hence thesis by A1,Def8;
end;
