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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 decode(tai64n):
 """ Convert TAI64N string to seconds since epoch. Note that dates before 2013 may not decode accurately due to leap second issues. If you need correct decoding for earlier dates you can try the tai64n package available from PyPI (U{https://pypi.python.org/pypi/tai64n}). @param tai64n: TAI64N-encoded time, as C{unicode}. @return: Seconds since UTC Unix epoch as C{float}. """

    seconds, nanoseconds = struct.unpack(_STRUCTURE, a2b_hex(tai64n[1:]))
    seconds -= _OFFSET
    return seconds + (nanoseconds / 1000000000.0)