Change the file position.

        The new position is specified by offset, relative to the
        position indicated by whence. Possible values for whence are:

            0: start of stream (default): offset must not be negative
            1: current stream position
            2: end of stream; offset must not be positive

        Returns the new file position.

        Note that seeking is emulated, so depending on the parameters,
        this operation may be extremely slow.
        )