'Implement L{IReactorFDSet.addWriter}.'

def addWriter(self, writer):

    fd = writer.fileno()
    if (fd not in self._writes):
        try:
            self._updateRegistration(fd, KQ_FILTER_WRITE, KQ_EV_ADD)
        except OSError:
            pass
        finally:
            self._selectables[fd] = writer
            self._writes.add(fd)
