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
read-write lock
A lock that lets many readers in at once, or one writer alone.
RwLock hands out many read guards or one write guard at a time; whether waiting writers block new readers depends on the operating system.