PRIMORDIUM

Built-in atoms (a/b/c/d/e/f/w/p) and wildcards (x/y/z) are reserved. New atoms get an uppercase letter or digit. Without rules they're inert decoration.

Rules fire AFTER built-in chemistry — they can never override or remove the seeded reactions, only add new behaviors. 2-input rules need two reactants; 3-input rules need three. Cases = probability denominator (1 = always, 100 ≈ 1%/encounter).

Every atom in the sim, in plain English. Built-ins are explained from first principles. Custom atoms get auto-generated descriptions from the rules you wrote — no AI, just templated translations of the math.