theorem Th74:
  the_rank_of M <= len M & the_rank_of M <= width M
proof
  ex P,Q st [:P,Q:] c= Indices M & card P = card Q & card P = the_rank_of
  M & Det EqSegm(M,P,Q)<>0.K by Def4;
  hence thesis by Th73;
end;
