Association list example, which is not working for some reason.

eg/assoc.castile

@@ -0,0 +1,36 @@
+struct assoc {
+  key: string;
+  value: string;
+  next: assoc|void;
+} for (singleton, update, lookup, remove)
+
+fun singleton(k: string, v: string) {
+  make assoc(key:k, value:v, next:null as assoc|void)
+}
+
+fun update(k: string, v: string, a: assoc) {
+  make assoc(key:k, value:v, next:a as assoc|void)
+}
+
+lookup : assoc, string -> string|void
+fun lookup(a: assoc, k: string) {
+  if a.key == k {
+    return a.value as string|void
+  }
+  n = a.next
+  typecase n is void {
+    return null as string|void
+  }
+  typecase n is assoc {
+    return lookup(n, k) as string|void
+  }
+}
+
+fun main() {
+  a = update("1", "first", update("2", "second", singleton("3", "third")));
+  r = lookup(a, "2");
+  print("um");
+  typecase r is void { print("NOT FOUND"); }
+  typecase r is string { print(r); }
+  print("ya");
+}