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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 write(self, fptr):
 """Write a channel definition box to file. """

        self._validate(writing=True)
        num_components = len(self.association)
        fptr.write(struct.pack('>I4s', 8 + 2 + num_components * 6, b'cdef'))
        fptr.write(struct.pack('>H', num_components))
        for j in range(num_components):
            fptr.write(struct.pack('>' + 'H' * 3,
                                   self.index[j],
                                   self.channel_type[j],
                                   self.association[j]))