interleave()

  choose a "pretty layout" method, e.g.

  More than one A?
  Y: remove two 'A' from pool
     return 'A' . interleave() . 'A'

  More than one B?
  Y: remove two 'B' from pool
     return 'B' . interleave() . 'B'

  one 'A'?
  Y: remove 'A' from pool
     return 'A'

  remove 'B' from pool
  return 'B'