Podelski, Andreas Programming Logics, MPI for Informatics, Max Planck Society;
https://rdcu.be/dwqhb (Publisher version)
Müller, M., Niehren, J., & Podelski, A. (1997). Ordering Constraints over Feature Trees. In G. Smolka (Ed.), Principles and Practice of Constraint Programming - CP97 (pp. 297-311). Berlin, Germany: Springer.