def get_spent(connection, transaction_id, condition_id):
    """Check if a `txid` was already used as an input.

    A transaction can be used as an input for another transaction. Bigchain
    needs to make sure that a given `txid` is only used once.

    Args:
        transaction_id (str): The id of the transaction.
        condition_id (int): The index of the condition in the respective
            transaction.

    Returns:
        The transaction that used the `txid` as an input else `None`
    """

    raise NotImplementedError
Check if a `txid` was already used as an input.

A transaction can be used as an input for another transaction. Bigchain
needs to make sure that a given `txid` is only used once.

Args:
transaction_id (str): The id of the transaction.
condition_id (int): The index of the condition in the respective
transaction.

Returns:
The transaction that used the `txid` as an input else `None`
bigchaindb/backend/query.py