reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;

theorem Th9:
  for A,B being set st A c= B holds varcl A c= varcl B
proof
  let A, B be set such that
A1: A c= B;
  B c= varcl B by Def1;
  then
A2: A c= varcl B by A1;
  for x,y st [x,y] in varcl B holds x c= varcl B by Def1;
  hence thesis by A2,Def1;
end;
