private void writeFieldNumber(int fieldNumber) throws IOException
    {
        if (numeric)
        {
            tail = packSink.packInt(fieldNumber, this, tail);
        }
        else
        {
            String fieldName = schema.getFieldName(fieldNumber);
            tail = packSink.packString(fieldName, this, tail);
        }

        mapSize++;
    }
Returns true if the last written field was a repeated field.
protostuff-msgpack/src/main/java/io/protostuff/MsgpackXOutput.java