theorem Th33:
  ERl(%O(Y)) = nabla Y
proof
  nabla Y c= ERl(%O(Y))
  proof
    let x1,x2 be object;
    assume
A1: [x1,x2] in nabla Y;
A2: Y in %O(Y) by TARSKI:def 1;
    x1 in Y & x2 in Y by A1,ZFMISC_1:87;
    hence thesis by A2,Def6;
  end;
  hence thesis;
end;
