theorem
  width G = m+1 & m>0 & k in Seg m implies Col(DelCol(G,width G),k) =
  Col(G,k) & k in Seg width DelCol(G,width G)
proof
  assume that
A1: width G = m+1 and
A2: m>0 and
A3: k in Seg m;
  k<=m by A3,FINSEQ_1:1;
  then
A4: k<width G by A1,NAT_1:13;
  1<=width G by A1,A2,SEQM_3:43;
  then
A5: width G in Seg width G by FINSEQ_1:1;
  1<=k by A3,FINSEQ_1:1;
  hence thesis by A1,A2,A5,A4,Th67;
end;
