code: @Override
  	public Iterator<String> getKeys(final String prefix) {
  		Set<String> keys = new HashSet<>();
  
  		for (String key : backing.keySet()) {
  			if (key.startsWith(prefix)) {
  				keys.add(key);
  			}
  		}
  
  		return keys.iterator();
  	}
docs: {@inheritDoc}