theorem Th12:
  for D being non empty set ex p being XFinSequence of D st len p = k
proof
  let D be non empty set;
  set y = the Element of D;
  set p = k --> y;
  reconsider p = k --> y as XFinSequence;
  reconsider p as XFinSequence of D;
  take p;
  thus thesis;
end;
