public Set<Object> keySet() {
        if (keySet == null) {
            keySet = new KeySet(this);
        }
        return keySet;
    }
Gets the keySet view of the map.
Changes made to the view affect this map.
To simply iterate through the keys, use {@link #mapIterator()}.

@return the keySet view
jphp-runtime/src/php/runtime/memory/support/ArrayMemoryMap.java