Create futures needed before starting the process of reading the
        address's value from the merkle tree.

        Args:
            addresses (list of str): addresses in the txn's inputs that
                aren't in any base context (or any in the chain).

def create_prefetch(self, addresses):
        """Create futures needed before starting the process of reading the
        address's value from the merkle tree.

        Args:
            addresses (list of str): addresses in the txn's inputs that
                aren't in any base context (or any in the chain).
        """

        with self._lock:
            for add in addresses:
                self._state[add] = _ContextFuture(address=add,
                                                  wait_for_tree=True)