theorem
  for F be FinSequence_of_Matrix of D st F={} holds block_diagonal(F,d)= {}
proof
  let F be FinSequence_of_Matrix of D;
  assume F={};
  then len Len F=0;
  then Len F=<*>REAL;
  then 0 =len block_diagonal(F,d) by Def5,RVSUM_1:72;
  hence thesis;
end;
