'Encode C{args} and write them to file.
C{kwargs} must contain the keywords C{fmt} and C{start}
@param args: The values to pack
@param fmt: See U{struct<http://docs.python.org/lib/module-struct.html>}
@param start: Position in file at which to write encoded data'

@safe
def pack(self, *args, **kwargs):

    encoded = struct.pack(kwargs['fmt'], *args)
    self._file.seek(kwargs['start'])
    self._file.write(encoded)
    self._file.flush()
