 Partial-list interleaving obstruction

No generic sequential pure Haskell operation can guarantee emission of every available input occurrence when either of two input spines may be the bottom value. To obtain an arbitrary element, it must eventually demand one input. That input can be undefined while the other is a defined singleton, so the demand blocks the promised singleton output. Choosing the other input first reverses the counterexample. Lazy evaluation cannot determine which computation will diverge; the positive two-stream fairness result needs proper lazy lists or a stronger concurrent computational model.