5.7.5 All Variables Used

[jlouis]

This task depends on Issue #107. Once that is done, then this is trivial to satisfy. You walk the document, and build a union set over the variables in it. A fragment contains a type signature which transitively knows about its variable uses.

Once the full set is created, you can use it to subtract from the set of variables. If that set becomes the empty set, then every variable has at least one use.