Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

fail-fast iterator

An iterator that throws as soon as it notices its collection changed underneath it.

Checks the collection's modification count on each next(); best effort only, so removing the second-to-last element can end a loop early without an exception.