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 Th19:
  for F being non empty finite set holds card F - 1 = card F -' 1
proof
  let F be non empty finite set;
  card F - 1 >= 0 by NAT_1:14,XREAL_1:48;
  hence thesis by XREAL_0:def 2;
end;
