Dear referees,

 we would like to thank for Your careful and valuable comments
and suggestions, which helped us to clarify the structure of the
paper and improved its content substantially.

 We followed all Your recommendations with one exception reflecting
the viewponts of both referees: we decided to keep the Appendix and
made more explicit the motivating role on the structure of our paper.

 We rectified the material around the introduction of quadratic forms
and the Arf invariant. This allowed the omission of Observation 2.8., 
Definition 2.9 and Theorem 5 (since they were not directly used in the 
proof of the main Theorem 1.) 
   
 All the best, Petr Somberg and Martin Loebl