theorem Th26:
  for x st x in X holds Class(nabla X,x) = X
proof
  let x such that
A1: x in X;
  now
    let y be object;
    assume y in X;
    then [y,x] in nabla X by A1,ZFMISC_1:87;
    hence y in Class(nabla X,x) by Th19;
  end;
  then for y being object holds y in X iff y in Class(nabla X,x);
  hence thesis by TARSKI:2;
end;
