
theorem Th25:
  for R being non empty RelStr, N being non empty Subset of R, B being set
  st B is_Dickson-basis_of N,R holds B is non empty
proof
  let R be non empty RelStr, N be non empty Subset of R, B be set;
  assume
A1: B is_Dickson-basis_of N,R;
  set a = the Element of N;
  ex b being Element of R st ( b in B)&( b <= a) by A1;
  hence thesis;
end;
