// Backtracking-enabled stacks (BStacks) are very useful for AI
// computation because they allow inserting a branching point anywhere
// in a computation, even deep inside nested function calls and loops.
//
// They normally operate serially, always exploring the main branch first
// and then optionally backtracking afterwards.
//
// However, the only thing preventing a BStack from backtracking in
// parallel to the main computation is the use of external mutable
// objects. With sufficient cloning and/or use immutable data structures,
// there is no obstacle to parallelizing.
//
// A further evolution of BStacks are PStacks. In PStacks, at any poin
// in time, every ongoing or potential computation is listed with a
// probability value p (usually 0 < p <= 1).
//
// Computations with higher probability are always pursued first.