Identifier fields which can be private are now private.
Identifier now inherits Ordered.
Define Undefined error, and its subclass SolverUndefinedError. Use it
mainly in solvers.

It used to be extremely bad: if the interesting failing test is the last
one (of 300), CEGISLike would test 1000*300 times before ordering it to
make it first.

Now we assume that interesting tests are discovered early.