[RFC] Element: limit to two elements?

[dourouc05]

For now, Element is used to retrieve as many elements from an array as the user wants: https://github.com/dourouc05/ConstraintProgrammingExtensions.jl/blob/master/src/ConstraintProgrammingExtensions.jl#L143-L168. Some solvers implement this form, but for instance CPLEX does not.

The problem comes from the error messages: I don't think it's possible to show an error to the user before reaching CPLEXCP.add_constraint (JuliaConstraints/CPLEXCP.jl@af0ee48).

Would it be better to limit Element to exactly two dimension (an index into the array and the destination variable)? Solvers could implement an optimisation if several Element sets are used, while having good user interface.