Lower level roles in Authorization Manager

Hello,
Is there  a way to enumerate lower level roles? I have a total of about 20 role definitions in the Authorization Manager. About 6-7 of them (intutively top level) "contain" one or more other ones (intutively lower level).

Now in the Role Assignments, I assign all the 20 roles, and add some Windows users to the top level. In the code when I use Roles.GetRolesForUser, I only receive a list of roles to which the user is actually assigned to.

My assumption was that if a user is assigned a role, and that role has lower level roles added to it, I should get a list of all of them programatically. Is there a way to do this?
Thanks.

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