theorem Th62:
 k in dom G implies Line(DelCol(G,i),k) = Del(Line(G,k),i)
proof
  set D = DelCol(G,i);
  assume that
A1: k in dom G;
  len D = len G by Def13;
  then
A2: dom D = dom G by FINSEQ_3:29;
  D.k = Del(Line(G,k),i) by A1,Def13;
  hence thesis by A1,A2,Th60;
end;
