Locant-min: heptane C-3,C-4 substituents ⇒ (3,4) < (4,5) lex-min

Sympy-exact witness of the IUPAC lowest-locant rule on a heptane (n = 7) parent chain bearing two substituents at positions 3 and 4. Forward numbering: locants (3, 4), sum = 7. Reverse numbering: (n+1−4, n+1−3) = (4, 5), sum = 9. …