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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 _format_str(format_int):
 """Return the string representation of a given numeric format."""

    for dictionary in _formats, _subtypes, _endians:
        for k, v in dictionary.items():
            if v == format_int:
                return k
    else:
        return 'n/a'