Pyrrole SEAr: N_{C2}/N_{C3} = 2 (resonance count); f_{C2} = 2/3 (statistical fraction)

Sympy-exact rational witness of pyrrole SEAr regioselectivity from resonance-structure enumeration. Setup: for electrophilic attack at a pyrrole ring position, count the number of independent resonance forms that stabilise the Wheland…