Remove permissions on library before adding new permissions

When a user submits a List item, my worklfow creates a new document library. I need to break permission inheritance on the newly created library, remove existing permissions, then set the permissions on the newly created document library where the list item's Created By (Author) user has Contribute.

I have successfully used the permissions.asmx web service to add permissions to the library.

Using the permissions.asmx web service, how do you first remove all existing permissions on the library? I assume I first need to use GetPermissionCollection, then RemovePermissionCollection. If so, does anyone have any examples of how to transform the GetPermissionCollection results to the membersIDxml parameter that the RemovePermissionCollection expects?