   Tree root:
      (cnt . node)

   Tree node:
      External symbol with the value: (key less more . value)  # 3 cells
   e.g.
      (key {5555} {7777} . {1111})

   Max DB-Size: 7 digits -> 2**42 (10**12) Blocks (2**48 Bytes, 256 TB)
      <Link> BEG TRANSIENT <key> EXTERN <7> EXTERN <7> DOT EXTERN <7> NIX
      -> keyLen: 30  # 6+2+30+1+7+1+7+2+7+1 = 64

                                             1 GB    64 GB    4 TB     256 TB
                                           ----------------------------------
   +Key  ("key" less more . obj)             39       36       33       30
   +Ref  (("key" . obj) less more . obj)     32       28       24       20
   +Idx  (("key" obj) less more . obj)       32       28       24       20
   +Sn  (("key" obj . T) less more . obj)    30       26       22       18
                                                     ----
