An 18-word ternary code exists with pairwise Hamming distance four or six.
Its distance distribution is A0=1, A4=15, A6=2. The nonnegative combination
(7/12)D1+(1/6)D2 is a Delsarte dual certificate equal to
18-(1+A4+A6), proving the matching upper bound.
This is an exact COMPUTED certificate, not an independently verified theorem.
