anchor.diff:  basic
       returns [JsonElement e]
       : id       { $e = $id.e;       }
  -    | company  { $e = $company.e;  }
  +    | seller   { $e = $seller.e;  }
       | buyer    { $e = $buyer.e;    }
       | date     { $e = $date.e;     }
       | products { $e = $products.e; }
  
positive.code: 
  receipt
      returns [JsonStructure e]
      @init      { $e = new JsonStructure("receipt"); }
      :
      ( basic { $e.add($basic.e); } )+
      ;
  
negative.code:     : '/'
      ;
  
  KEYWORD_BUYER
      : 'buyer' | 'comprador'
      ;
  
  KEYWORD_SELLER
      : 'seller' | 'vendedor'
      ;
  
  KEYWORD_TAX
      : 'tax' | 'juros'
      ;
  