Comment by masswerk

So have

  (USAGE: PALIN A B B A ...)
  START

at the beginning and a MEMORY rule (must have exactly 4 entries) at the end:

  (MEMORY XYZZY
       (0 = A)
       (0 = B)
       (0 = A A)
       (0 = B B)
  )

:-)

PS: I'm not sure, if the initial welcome message is actually mandatory, but the MEMORY rule definitely is.