reserve x, y for set;
reserve D for non empty set;
reserve UN for Universe;

theorem Th2:
  op2.({},{}) = {} & op1.{} = {} & op0 = {}
proof
  [{},{}] in [:{{}},{{}}:] by ZFMISC_1:28;
  hence op2.({},{}) = {} by FUNCT_2:50;
  {} in {{}} by TARSKI:def 1;
  hence op1.{} = {} by FUNCT_2:50;
  thus thesis;
end;
