Status: IN
Sentinel initial values (-1, 0, None) bootstrap the O(1)-space running accumulators that power single-pass streaming, encoding boundary conditions directly into loop initializers so the first iteration executes the same code path as all subsequent iterations.
Sentinels and accumulators are co-dependent: accumulators need correct initial state to avoid first-iteration branching, and sentinels exist precisely to provide that state — neither pattern works well without the other
Depends on (SL): sentinel-initialization-encodes-boundary-conditions, o1-space-via-running-accumulators