<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ApprovalUrnBuilder(subject, user, approval_id):
 """Encode an approval URN."""

    return aff4.ROOT_URN.Add("ACL").Add(subject).Add(user).Add(approval_id)