TODO: This comment seems to be partly out of date.
terminates cs checks if the functions represented by cs
terminate. The call graph cs should have one entry (Call) per
recursive function application.
Right perms is returned if the functions are size-change terminating.
If termination can not be established, then Left problems is
returned instead. Here problems contains an
indication of why termination cannot be established. See lexOrder
for further details.
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).