UnityTransformRotation.cs

    // Unity C# extension methods to the Transform class.
    // Transform already has TransformPosition/InverseTransformPosition.
    // Add the correct Quaternion terms to apply or find a relative rotation.

    public static Quaternion TransformRotation(this Transform transform,
        Quaternion localRotation)
    {
        return transform.rotation * localRotation;
    }

    public static Quaternion InverseTransformRotation(this Transform transform,
        Quaternion rotation)
    {
        return Quaternion.Inverse(transform.rotation) * rotation;
    }