A A
  A B
  A C
  A D
B
  B A
  B B
  B C
C
  C A
  C B
  C C
  C D
  C E
D
  D A
    D A A
    D A B
    D A C
    D A D
  D B
  D C
  D D
    D D A
    D D B
    D D C
    D D D
E
  E A
  E B
F
  F A
  F B
  F C
  F D
  F E
  F F
  F G
  F H
  F I
  F J
  F K
  F L
  F M
  F N
  F O
  F P
  F Q