Termination checker, based on "A Predicative Analysis of Structural Recursion" by Andreas Abel and Thorsten Altenkirch (JFP'01), and "The Size-Change Principle for Program Termination" by Chin Soon Lee, Neil Jones, and Amir Ben-Amram (POPL'01).
TODO: This comment seems to be partly out of date.
is returned if the functions are size-change terminating.
Note that this function assumes that all data types are strictly positive.
The termination criterion is taken from Jones et al. In the completed call graph, each idempotent call-matrix from a function to itself must have a decreasing argument. Idempotency is wrt. matrix multiplication.
This criterion is strictly more liberal than searching for a lexicographic order (and easier to implement, but harder to justify).