private static void doubleXYZT(PartialXYZT partialXYZT, XYZT p) {
    doubleXYZ(partialXYZT, p.xyz);
  }
Doubles {@code p} and puts the result into this PartialXYZT.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java