=== checkback after 1nt rebid ===
we play double checkback (FIXME even by passed hand?)

  * (special sequence) after 1m - 1H - 1NT - 2S is 44 (exactly) majors, INV (NF) 
  * 2NT is transfer to clubs
  * 2C is puppet to 2D, usually with INV hands. natural developments.
  * 2D is game forcing, natural developments.