A bit of a simple system I put together for having tiered skills. The skills that I have as being tiered are otherwise unavailable to anyone. They're "unlocked" when a character's prerequisite skill reaches 100%. Theoretically you can use this to have an endless hierarchy of skills, and could probably be feasably expanded to creating multiple prerequisite skills to "unlock" the higher tier skills… which I may do myself. Anyway, have at it, break it, but don't buy it. And certainly don't bother me about it if it doesn't work for you.