Constraint syntax is not expressive enough.

Here is a property that we would quite like the solver to have:

> given any solver result/solution we should be able to construct a set of solver input constraints that uniquely determine the same result (in any environment in which that solution exists)

Or to put it another way, we should be able to freeze the result of the solver and feed it back into the solver later to get the same result. The solver almost has this property but not quite. In particular the introduction of setup deps and qualified goals means that simple "pkg == version" constraints are no longer expressive enough to describe all the solutions the solver can produce. If the solver picks two versions of package foo (e.g. one a "main" lib package and another as a setup dep) then we cannot just constrain "foo == 1.0", we would need to specify the scope of the constraint. Notice also that "primary" vs "setup" dep scope is not enough, since we could pick two versions of foo both as setup deps. Probably what is required to express all the solutions is something corresponding to the internal goal qualifications.