Bergstra, J. A., Bethke, I., & Rodenburg, P. H. (1995). A propositional logic with four values: true, false, divergent and meaningless. Journal of Applied Non-Classical Logics, 5, 199-217.
van Wamel, J. J. (1995). Verification Techniques for Elementary Data Types and Retransmission Protocols. [Thesis, fully internal, Universiteit van Amsterdam].