  The present note refers to a result proposed in [Elhadj and Sprott, 2012],
and shows that the Theorem therein is not correct. We explain that a proof of
that Theorem cannot be given, as the statement is not correct, and we underline
a mistake occurring in their proof.
  Since this note is supplementary to [Elhadj and Sprott, 2012], the reader
should consult this paper for further explanations of the matter and the
symbols used.
