Permutations and Combinations
Counting for GA: multiplication and addition principles, nPr and nCr with their identities, arrangements with restrictions (together, apart, fixed position, gap method), circular arrangements and necklaces, identical objects, selections with at-least conditions by cases and by complement, distributions of identical and distinct objects, grouping, derangements, digit formation and geometric counts. Each formula is checked by brute-force enumeration in the verification script.
