$rx<T>  constructor 
Implementation
factory $rx(T val, {String? ref, List<String>? tags})
{
  if(_cache.containsKey(ref))
  {
    return _cache[ref] as $rx<T>;
  } else {
    final $rx<T> _rx = $rx._internal(val, ref:ref, tags:tags);
    _cache[ref] = _rx;
    return _rx;
  }
}