What I would say just if   
      
    there is the *only* rule   
      
   rule(abs(x)==-x if x in Float x<0)   
      
   If x<0 abs(x) can be substituted from -x   
   else remain abs(x)   
   But possible I not have experience in rules on math formulas in CAS programs   
   etc... Thank you   
   Good morning...   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)