theorem :: GOBOARD7:9

for i, j being Nat
for G being Go-board st 1 <= i & i + 1 <= len G & 1 <= j & j + 1 <= width G holds
(1 / 2) * ((G * (i,j)) + (G * ((i + 1),(j + 1)))) = (1 / 2) * ((G * (i,(j + 1))) + (G * ((i + 1),j)))