==== checkback after 2NT rebid ====
1m - 1M - 2NT. this 2NT is alertable, we include here hands like
  * 18-20 HCP balanced. may have fit3 or fit4 for responder major
  * 18(17)+ with 5m-4M-22
  * 18(17)+ with minor onesuiter (stronger than 1m - x - 3m)

thus, 3C is a special relay for clarification:
  * 3D = minor onesuiter. responder may rebid his major to search a 5-3 fit
  * 3H/S/NT = 18-20 BAL. up the line. if it's fit, it is 3 card fit. if it's the other major, it is a 4 card suit.
  * 4m (opened minor) = fit4, 5422 (5 in opened minor, fit4 for responder's major)
  * 4om (the other minor) = fit4, 4432
  * 4M (fitting responder major) = fit4, 4333

other than this 3C relay, the responder may bid
  * 3D = ART, fit for opener minor. the responder major in exactly 4 cards, no interest in searching a 53 fit.
  * 1m - 1H - 2NT - 3H (rebid) = ? 6 cards GF ? cannot be very weak (1m - 2H then)
  * 1m - 1H - 2NT - 3S = ? 44 similar with the rebid after 1NT ?
  * 1m - 1S - 2NT - 3H = ? 55 ? with 54 simply use the 3C relay
  * 1m - 1S - 2NT - 3S (rebid) = ? 6 cards GF ?