docs: def depart(self):
          """
          Remove the current remote unit, for the active hook context, from
          this conversation.  This should be called from a `-departed` hook.
          """
          unit = hookenv.remote_unit()
          self.units.remove(unit)
          if self.units:
              unitdata.kv().set(self.key, self.serialize(self))
          else:
              unitdata.kv().unset(self.key)