<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gather(self):
 """ Interact with segy in gather mode Returns ------- gather : Gather """

        if self.unstructured:
            raise ValueError(self._unstructured_errmsg)

        if self._gather is not None:
            return self._gather

        self._gather = Gather(self.trace, self.iline, self.xline, self.offsets)
        return self._gather