Currently register looks like:

   private static <T> void register(Map<T, T> substitutions, T annotated, T original, T target) {
        if (annotated != null) {
            guarantee(!substitutions.containsKey(annotated) || substitutions.get(annotated) == original || substitutions.get(annotated) == target, "Already registered: %s", annotated);
            substitutions.put(annotated,

If possible, please provide a link(s) to the language documentation to prove its existence:

• https://github.com/grain-lang/grain

If possible, please provide a code snippet of Hello World in the language:

...