private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException {
variables = new HashMap<AttributeName, PositionalNodeType>();
int elements = input.readInt();
for (int i = 0; i < elements; i++) {
AttributeName attribute = (AttributeName) input.readObject();
PositionalNodeType node = (PositionalNodeType) input.readObject();
variables.put(attribute, node);
}
}