theorem
  width G = m+1 & m>0 & k in Seg m implies Col(DelCol(G,1),k) = Col(G,k+
  1) & k in Seg width DelCol(G,1) & k+1 in Seg width G
proof
  assume that
A1: width G = m+1 and
A2: m>0 and
A3: k in Seg m;
  1<=width G by A1,A2,SEQM_3:43;
  then
A4: 1 in Seg width G by FINSEQ_1:1;
  1<=k & k<=m by A3,FINSEQ_1:1;
  hence thesis by A1,A4,Th68;
end;
