@Override
            protected void mergeFrom(Input input, T message) throws IOException
            {
                final Object value = input.mergeObject(message, schema);
                if (input instanceof GraphInput
                        && ((GraphInput) input).isCurrentMessageReference())
                {
                    // a reference from polymorphic+cyclic graph deser
                    Collection<Object> existing = accessor.get(message);
                    if (existing == null)
                        accessor.set(message, existing = messageFactory.newMessage());
                    
                    existing.add(value);
                }
            }
For lazy initialization called by {@link RuntimeFieldFactory}.
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeRepeatedFieldFactory.java