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

open-closed principle

Adding a new kind should mean adding code, not editing code that already works.

Code is open for extension and closed for modification when new variants arrive as new implementations of an interface; a sealed type with exhaustive switches makes the opposite trade-off.