Modelling

The best account of our modelling language ESRA, which is short for Executable Symbolism for Relational Algebra, is in [FPÅ+04]:

• ESRA is high-level. ESRA supports the use of relation decision variables, as originally advocated in [Fle01] and common in software engineering (cf. UML diagrams) and database design (cf. entity-relation-attribute diagrams). A prior design of ESRA supports the use of function decision variables, as originally advocated in [FHK01b]. In the latter publication, we also advocated sequence and permutation decision variables.

• ESRA is solver independent, and even optimisation-technology independent, as nothing prevents the writing of backends for any constraint processing solver, (constraint-based) local-search solver, or mathematical programming engine, for instance. A prototype compiler of ESRA into SICStus Prolog is described in [Nor06], while a compiler of the functional variant of ESRA into OPL is described in [Wra02, Hni03a].

Relation decision variables allow an implementation-neutral modelling of many concepts that are usually modelled as matrices of decision variables [FFH+02b].