Act on the following rule :
            items : items item
                  | item

def _act_on_list(self, lhs):
        '''
        Act on the following rule :
            items : items item
                  | item
        '''
        lhs[0] = []
        if len(lhs) == 3:
            lhs[0] = lhs[1]
        # lhs[len(lhs)-1] may be different from lhs[-1]
        # Yacc use some internal method to get the element, see yacc.py:240
        item = lhs[len(lhs) - 1]
        if item:
            lhs[0].append(item)