  It is shown that the extended quadratic residue code of length 20 over GF(7)
is a unique self-dual [20,10,9] code C such that the lattice obtained from C by
Construction A is isomorphic to the 20-dimensional unimodular lattice D_{20}^+,
up to equivalence. This is done by converting the classification of such
self-dual codes to that of skew-Hadamard matrices of order 20.
