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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 _find(string, sub_string, start_index):
 """Return index of sub_string in string. Raise TokenError if sub_string is not found. """

    result = string.find(sub_string, start_index)
    if result == -1:
        raise TokenError("expected '{0}'".format(sub_string))
    return result