http://idk.i2p/javadoc-i2p/net/i2p/crypto/eddsa/math/GroupElement.html
$r = p + q$ where $p = this = (X1 : Y1 : Z1 : T1), q = (q.X, q.Y, q.Z, q.T) = (Y2 + X2, Y2 - X2, Z2, 2 * d * T2)$ $r$ in $P \times P$ representation: $X' = (Y1 + X1) * (Y2 + X2) - (Y1 - X1) * (Y2 - X2)$ $Y' = (Y1 + X1) * (Y2 + X2) + (Y1 - X1) * (Y2 - X2)$ $Z' = 2 * Z1 * Z2 + 2 * d * T1 * T2$ $T' = 2 * Z1 * T2 - 2 * d * T1 * T2$ Setting $A = (Y1 - X1) * (Y2 - X2), B = (Y1 + X1) * (Y2 + X2), C = 2 * d * T1 * T2, D = 2 * Z1 *...