<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def list_rds(region, filter_by_kwargs):
 """List all RDS thingys."""

    conn = boto.rds.connect_to_region(region)
    instances = conn.get_all_dbinstances()
    return lookup(instances, filter_by=filter_by_kwargs)