Roles and permissions

Hello everyone,

I have just noticed that there is no Delete button to delete Student, Guest, and Authenticated User role in Site Administration - Users - Permissions - Define roles. But instead there is Delete button for higher level roles such as Administrator, Course Creator, Teacher, and Non-Editing Teacher.

My expectation was contrary. There should no possibilities to delete Administrator role since this role is very important.

Anyone can give me some overview? I'm using Moodle 1.8.1.

Thank you,
Soben