debug function

def pprint(self, indent='', f=None):
        """debug function"""
        if self.arg is not None:
          print(indent + util.keyword_to_str(self.keyword) + " " + self.arg)
        else:
          print(indent + util.keyword_to_str(self.keyword))
        if f is not None:
             f(self, indent)
        for x in self.substmts:
            x.pprint(indent + ' ', f)
        if hasattr(self, 'i_children') and len(self.i_children) > 0:
           print(indent + '--- BEGIN i_children ---')
           for x in self.i_children:
               x.pprint(indent + ' ', f)
           print(indent + '--- END i_children ---')