theorem Th36:
  x = <* P, Q, R, S *> implies
  cross-ratio(P,Q,R,S) = cross-ratio-tuple(x)
  proof
    assume x = <* P, Q, R, S *>;
    then x.1 = P & x.2 = Q & x.3 = R & x.4 = S;
    hence thesis by Def03;
  end;
