For me the result of rule, it can be error only if there is find one antinomy   
      
   m:=rule(...)   
      
   m(expression)   
   Should return the same expression (if the rule can not be applied)   
   Or   
   the expression apply rule produce   
   Or   
   Error if it find something as 1=2   
   something it is not possible   
   [this mean that the input is wrong or rules produce antinomy ]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)