Add self-referential nodes to egraph

[silversquirl]

Is it possible to add an ENode that references itself? Or, more generally, to add a cycle to the EGraph?
It seems like it must be doable since egg itself creates self-references using rewrites, but I'm not sure how to do it with the user-facing API.

[mwillsey]

Yes! You cannot directly make a cycle, but you can do it with a union. Here is some (un-parsed) code to demonstrate:

let x = egraph.add(Leaf("x"));
let node = egraph.add(Node(x, x));
egraph.union(x, node);