reserve i,j,x,y for object,
  f,g for Function;
reserve T,T1 for finite Tree,
  t,p for Element of T,
  t1 for Element of T1;

theorem
  for x being set holds card root-tree x = 1
proof
  let x be set;
  root-tree x = {[{},x]} by TREES_4:6;
  hence thesis by CARD_1:30;
end;
