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 Th19:
  |(-p1, p2)| = -|(p1, p2)|
proof
  |(-p1, p2)| = |((-1)*p1, p2)|
    .= (-1)*|(p1, p2)| by Th17;
  hence thesis;
end;
