Status: IN
NullIterator's always-done semantics completes the recursive triad's flexible traversal infrastructure by eliminating special-case handling at leaf boundaries: without NullIterator, every traversal algorithm must distinguish leaves from composites, but with it, the dual design axes (control and robustness) operate uniformly across all tree nodes regardless of structural position.
NullIterator is the "last mile" mechanism making the recursive triad's traversal truly uniform
Depends on (SL): null-iterator-enables-composite-transparency, recursive-triad-traversal-flexibility