datatype bbResult = CORRECT | PENDING | INCORRECT;

datatype 'a bbSearch = BBEmpty
                     | BBValue of 'a
                                * (unit -> 'a bbSearch)
                                * (unit -> 'a bbSearch);

fun bbSolve bbPred BBEmpty                         = []
  | bbSolve bbPred (BBValue(s, increment, extend)) =
       case bbPred s of
            CORRECT   => s::(bbSolve bbPred (increment ()))
          | PENDING   => bbSolve bbPred (extend ())
          | INCORRECT => bbSolve bbPred (increment ());

fun bbProducer _     _     NONE     = BBEmpty
  | bbProducer bbinc bbext (SOME s) =
        BBValue(s,
                fn () => bbProducer bbinc bbext (bbinc s),
                fn () => bbProducer bbinc bbext (bbext s));