@Override
        public void writeTo(Output output, Entry<K, V> message) throws IOException
        {
            final K key = message.getKey();
            final V value = message.getValue();
            if (key != null)
                writeKeyTo(output, 1, key, false);

            if (value != null)
                writeValueTo(output, 2, value, false);
        }
The pipe schema of the {@link Map}.
protostuff-collectionschema/src/main/java/io/protostuff/MapSchema.java