Add note in is_not_zero example.

Closes #297

src/macros.rs

@@ -281,6 +281,9 @@ impl Applier<SimpleLanguage, ()> for MySillyApplier {
 fn is_not_zero(var: &'static str) -> impl Fn(&mut EGraph, Id, &Subst) -> bool {
     let var = var.parse().unwrap();
     let zero = SimpleLanguage::Num(0);
+    // note this check is just an example,
+    // checking for the absence of 0 is insufficient since 0 could be merged in later
+    // see https://github.com/egraphs-good/egg/issues/297
     move |egraph, _, subst| !egraph[subst[var]].nodes.contains(&zero)
 }
 ```