theorem Th77:
  n = 0 or m = 0 implies the_rank_of Segm(M,nt,mt) = 0
proof
  set S=Segm(M,nt,mt);
  assume n=0 or m = 0;
  then len S=0 or width S=0 by Th1,MATRIX_0:def 2;
  hence thesis by Th74;
end;
