theorem Th14:
  {x} is constituted-DTrees iff x is DecoratedTree
proof
  thus {x} is constituted-DTrees implies x is DecoratedTree
  proof
    assume
A1: for y st y in {x} holds y is DecoratedTree;
    x in {x} by TARSKI:def 1;
    hence thesis by A1;
  end;
  assume
A2: x is DecoratedTree;
  let y;
  thus thesis by A2,TARSKI:def 1;
end;
