reserve x,y,z for object, X,Y for set,
  i,k,n for Nat,
  p,q,r,s for FinSequence,
  w for FinSequence of NAT,
  f for Function;

theorem Th33:
  <*x,y*> is DTree-yielding iff x is DecoratedTree & y is DecoratedTree
proof
A1: x is DecoratedTree & y is DecoratedTree iff {x,y} is constituted-DTrees
  by Th17;
  rng <*x,y*> = {x,y} by FINSEQ_2:127;
  hence thesis by A1;
end;
