class TokenIterator(object):
    def __init__(self, tokens):
        self._tokens = tokens
        self._index = 0
    
    def next_value(self, token_type):
        return self._next(token_type).value
    
    def _next(self, token_type):
        token = self._tokens[self._index]
        if token.type == token_type:
            self._index += 1
            return token
        else:
            raise self._unexpected_token_type(token_type, token)
    
    def skip(self, token_type, token_value):
        token = self._tokens[self._index]
        if token.type == token_type and token.value == token_value:
            self._index += 1
            return True
        else:
            raise self._unexpected_token_type(token_type, token)
    
    def try_skip(self, token_type, token_value):
        token = self._tokens[self._index]
        if token.type == token_type and token.value == token_value:
            self._index += 1
            return True
        else:
            return False