<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def register(id, url=None):
 """Register a UUID key in the global S3 bucket."""

    bucket = registration_s3_bucket()
    key = registration_key(id)
    obj = bucket.Object(key)
    obj.put(Body=url or "missing")
    return _generate_s3_url(bucket, key)