theorem Th24:
  <* <* d1,d2,d3 *> *> is Matrix of 1,3,D
  proof
A1: <* d1,d2,d3 *> is FinSequence of D by FINSEQ_2:14;
    len <* d1,d2,d3 *> = 3 by FINSEQ_1:45;
    hence thesis by A1,MATRIX_0:11;
  end;
