void copyConditional(CachedXYT other, int icopy) {
      Curve25519.copyConditional(yPlusX, other.yPlusX, icopy);
      Curve25519.copyConditional(yMinusX, other.yMinusX, icopy);
      Curve25519.copyConditional(t2d, other.t2d, icopy);
    }
If icopy is 1, copies {@code other} into this point. Time invariant wrt to icopy value.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java