reserve i,j,k,l for natural Number;
reserve A for set, a,b,x,x1,x2,x3 for object;
reserve D,D9,E for non empty set;
reserve d,d1,d2,d3 for Element of D;
reserve d9,d19,d29,d39 for Element of D9;
reserve p,q,r for FinSequence;

theorem
  i in Seg l implies i in Seg(l+j)
proof
  l <= l+j by NAT_1:11;
  then Seg l c= Seg(l+j) by FINSEQ_1:5;
  hence thesis;
end;
