TODO
====
A list of things to do for Speculate.
* remove lexicompare as it is already exported by Express
* simpilfy "bench" handing. Do like in LeanCheck and Extrapolate.
* (code readability) review and document code
* (code readability) rename semi to inqualities everywhere.
* (interface) print errors on stderr, not on stdout
* (performance) note that variable assignments form a lattice. So I only
need to test stuff from upper if the lower is true. Of course, testing is
the expensive thing. But it does not pay off to test x + y = z + w before
testing x + y = y + x. The second needs to hold for the first to hold. And,
it will be far more common!