They count the crystal’s #genus carefully, blowing smoke through tiny holes to see which tunnels run true. Spells fail if they don’t describe things with a mathematical precision that feels impossible without computers– but the moon’s full and the Dark Lord is waiting.
In mathematics, genus (plural genera) has a few different, but closely related, meanings. The fastest, easiest and most intuitive way to introduce the concept of a genus is that it is the number of “holes” of a surface. A sphere has genus 0, and a torus has genus 1. https://en.wikipedia.org/wiki/Genus_(mathematics)