Grammar: (Combine (Combine (Group a b) c) d)
Input: c d a b;

  c ::-> (Choice (Combine d (Group a b)) Empty)
  d ::-> (Choice (Group a b) Empty)
  a ::-> b
  b ::-> Empty

Input a b c d

  a ::-> (Group b (Choice Empty (Combine c d)))
  b ::-> (Choice Empty (Combine c d))
  c ::-> (Choice Empty d)
  d ::-> Empty

Input d a b c

  d ::-> (Choice Empty (Combine (Group a b) c))
  a ::-> (Group b (Choice Empty d))
  b ::-> (Choice Empty d)
  c ::-> Empty

Grammar (Combine (Combine (Combine a b) c) d)
Input: d a

  d ::-> (Choice (Combine (Combine a b) c) Empty)
  a ::-> (Choice (Combine b c) Empty)