An example of substituability test that is recursive

[John Rose]

On Jan 31, 2019, at 6:34 AM, forax at univ-mlv.fr wrote:
> 
> The other solution is to say that == should do an upcall to equals (after the null checking and the class checking), if equals throw a StackOverflow, it's the expected behavior because the user is in control of that behavior.

What you are doing here, I think, is exposing a requirement
that we *don't* use the control stack for recursion on subst.
testing (or hashing).  That's a reasonable requirement.
It leads to a worklist algorithm for doing this tricky thing,
just like we had to do many times in the JIT.