theorem Th24:
  dist(p,q) = dist(-p,-q)
proof
  thus dist(p,q) = dist(q-q,p-q) by Th23
    .= dist(q-q,p+-q)
    .= dist(|[0,0]|,p+-q) by EUCLID:54,RLVECT_1:5
    .= dist(p-p,p+-q) by EUCLID:54,RLVECT_1:5
    .= dist(p+-p,p+-q)
    .= dist(-p,-q) by Th21;
end;
