
theorem Th15:
  for L,R being RelStr for x,y being Element of Sub L st y = R
  holds x <= y iff x is SubRelStr of R
proof
  let L,R be RelStr;
  let x,y be Element of Sub L;
  x <= y iff ex R being RelStr st y = R & x is SubRelStr of R by Def2;
  hence thesis;
end;
