Eerke A. Boiten John Derrick Gerhard Schellhorn Relational concurrent refinement part II: Internal operations and outputs. 65-102 2009 21 Formal Asp. Comput. 1-2 http://dx.doi.org/10.1007/s00165-007-0066-z db/journals/fac/fac21.html#BoitenDS09