1. A method comprising: discovering a plurality of roles, wherein each of said roles is a biclique covering of a set of vertices of a first type and a corresponding set of vertices of a second type; selecting a first role in said plurality and finding all roles in said plurality that have a set of vertices of said second type that is a subset of said set of vertices of said second type in said first role; removing each of said subsets from said set of vertices of said second type corresponding to said first role; and reassigning said vertices of said first type to said roles such that original associations between said vertices of said first type and said vertices of said second type are maintained.