theorem
  <:root-tree x, root-tree y:> = root-tree [x,y]
proof reconsider x9 = x as Element of {x} by TARSKI:def 1;
  reconsider y9 = y as Element of {y} by TARSKI:def 1;
   (root-tree [x9,y9])`1 = root-tree x & (root-tree [x9,y9])`2 = root-tree y
  by Th25;
  hence thesis by TREES_3:40;
end;
