Get the account state for a token type at the given (block_id,vtxindex), plus the account state for
    the same token type at the prior operation.

    The current account must have been debited or credited at (block_id,vtxindex)

    Returns {'current': ..., 'prior': ...} on success
    Returns None on error

def namedb_get_account_delta(cur, address, token_type, block_id, vtxindex):
    """
    Get the account state for a token type at the given (block_id,vtxindex), plus the account state for
    the same token type at the prior operation.

    The current account must have been debited or credited at (block_id,vtxindex)

    Returns {'current': ..., 'prior': ...} on success
    Returns None on error
    """
    sql = 'SELECT * FROM accounts WHERE address = ? AND type = ? AND (block_id <= ? OR block_id == ? AND vtxindex <= ?) ORDER BY block_id DESC, vtxindex DESC LIMIT 2;'
    args = (address, token_type, block_id, block_id, vtxindex)

    rows = namedb_query_execute(cur, sql, args)
    data = []
    for r in rows:
        tmp = {}
        tmp.update(r)
        data.append(tmp)

    if len(data) != 2:
        return None

    # the current account state must land on this block_id/vtxindex
    if data[0]['block_id'] != block_id or data[0]['vtxindex'] != vtxindex:
        return None

    return {'current': data[0], 'prior': data[1]}