TABLE learned(switchid, portid, macaddr);

ON packet(pkt):
  INSERT (pkt.locSw, pkt.locPt, pkt.dlSrc) INTO learned;

  DELETE (pkt.locSw, pt, pkt.dlSrc) FROM learned WHERE
    not pt = pkt.locPt;

  DO forward(new) WHERE
    learned(pkt.locSw, new.locPt, pkt.dlDst)
    OR
    (NOT learned(pkt.locSw, ANY, pkt.dlDst) AND
     pkt.locPt != new.locPt);

ON switch_down(swd):
  DELETE (swd.sw, ANY, ANY) FROM learned;