<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _next_offset(self):
 """Return the offset of the next line to read."""

    if self._filestream:
      offset = self._filestream.tell()
      if offset:
        offset -= 1
    else:
      offset = self._initial_offset

    return offset