Ralph Martin and Tamas Varady would like to acknowledget he support of the European Union for their work into reversee ngineeringu nder Copernicus grant RECCAD 94-1068.T he authors would like to thank to the memberso f the Copernicust eamf or many helpfuld iscussionsT. he National ScienceF oundationo f the HungarianA cademyo f Sciencesa lso supportedt his work under grant 16420;