theorem
 not i in Seg width M implies DelCol(M,i) = M
 proof assume
A1: not i in Seg width M;
A2: len DelCol(M,i) = len M by Def13;
  for k st 1 <= k & k <= len M holds M.k = DelCol(M,i).k
   proof let k such that
A3: 1 <= k & k <= len M;
A4:   k in dom M by A3,FINSEQ_3:25;
      len Line(M,k) = width M by Def7;
      then
A5:   not i in dom Line(M,k) by A1,FINSEQ_1:def 3;
      thus M.k = Line(M,k) by A4,Lm1
        .= Del(Line(M,k),i) by A5,FINSEQ_3:104
        .= DelCol(M,i).k by A4,Def13;
   end;
 hence DelCol(M,i) = M by A2,FINSEQ_1:14;
 end;
