reserve i, n for Nat,
  x, y, a for Real,
  v for Element of n-tuples_on REAL,
  p, p1, p2, p3, q, q1, q2 for Point of TOP-REAL n;

theorem
  |(p, q1-q2)| = |(p, q1)| - |(p, q2)| by Th22;
