Hückel π-counts: benzene 6 (mod 4 = 2, aromatic); cyclobutadiene 4 (mod 4 = 0, anti-aromatic)

Sympy-exact witness of Hückel classification for three canonical heterocyclic and carbocyclic systems. Benzene: π=6, 6 mod 4 = 2, aromatic flag = 1. Cyclobutadiene: π=4, 4 mod 4 = 0, aromatic flag = 0 (anti-aromatic). Pyrrole (π=6…