raf.writeShort(a.size() & 0xFFFF);
for (String key : a.keySet()) {
String alias = a.get(key);
byte key_size = (byte)key.length();
byte alias_size = (byte)alias.length();
raf.writeByte(key_size & 0xFF);
raf.write(key.getBytes("UTF-8"), 0, key_size);
raf.writeByte(alias_size & 0xFF);
raf.write(alias.getBytes("UTF-8"), 0, alias_size);
block_size += 2 + key_size + alias_size;
}