theorem Th25:
  <* <* d1 *> ,<* d2 *> ,<* d3 *> *> is Matrix of 3,1,D
  proof
    reconsider p = <* d1 *>, q = <* d2 *>, r = <* d3 *> as FinSequence of D;
    len p = 1 & len q = 1 & len r = 1 by FINSEQ_1:40;
    hence thesis by MATRIXR2:34;
  end;
