This is a small example implementation of a websockets client to subscribe to ledger transactions.
To stream the full ledger from the beginning:
python watcher.py
To stream the transaction of an account address:
python watcher.py <account>
The method _load_last_state_version in Watcher can be used to load the
state version of the last received and stored transaction from e.g. a database or disk
(depending whether and where you store and process transactions in transaction_handler).
However, be careful: a stored state version is account dependent and different accounts need to have their own stored state version!