anchor.diff:      ;
   
   clause 
  -    : (predicate '.') | (predicate ':-') (predicatelist '.')
  +    : (predicate '.')
  +    | (predicate ':-') (predicatelist '.')
       ;
   
   predicatelist 
  
positive.code: grammar prolog;
  
  program 
      : clauselist query?
      ;
  
  clauselist 
  
negative.code:     ;
  
  predicate 
      : atom
      | atom '(' termlist ')'
      ;
  
  termlist 
      : term ( ',' term )*
      ;
  
  term 
      : VARIABLE
      | atom
      | integer
      | FLOAT
      | structure
      ;
  
  structure 
  