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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _iterate_subsequences(self, tokens):
 """ Using regex invokes this function, which significantly impacts performance of adapt. it is an N! operation. Args: tokens(list):
 list of tokens for Yield results. Yields: str: ? """

        for start_idx in xrange(len(tokens)):
            for end_idx in xrange(start_idx + 1, len(tokens) + 1):
                yield ' '.join(tokens[start_idx:end_idx]), start_idx