 Formally, the link between ranges and correlations described above may be approached by using a restricted product state space as a model of the architecture of the system (van Benthem 2006, van Benthem and Martinez 2008). The basic structures are constraint models, versions of which have been around in the literature for some years (for example Fagin et al. 1995 in the study of epistemic logic, and Ghidini and Giunchiglia 2001 in the study of context dependent reasoning). Constraint models have the form 