SQL Server - How to list logins assigned to a credential?

SQL Server - How to list logins assigned to a credential?

A SQL Server credential is used to allow SQL Server User (not Windows
User) to have an identity outside of the SQL Server and to enable them
performing various tasks.
From the http://technet.microsoft.com/en-us/library/ms189828.aspx site,
credential can be added to a (SQL Server) Login as follows:
ALTER LOGIN Mary5
ADD CREDENTIAL EKMProvider1;
GO
My question is, where can I see in the system tables/views the
Logins/Users that have been assigned to a given credential? Thanks!