theorem Th25:
  dist(q-q1,q-q2) = dist(q1,q2)
proof
  -(q-q1)= q1-q & -(q-q2) = q2-q by RLVECT_1:33;
  hence dist(q-q1,q-q2) = dist(q1-q,q2-q) by Th24
    .= dist(q1,q2) by Th23;
end;
