“In turning now to the principle of dialogue underlying the Constitution's structure, it is important first to note a basic distinction. The Constitution's structural theory rests on two closely related but nevertheless separate principles: separation of powers and checks and balances. The first principle requires that the branches of government be identifiably discrete. The second assumes that the branches are separate and then concentrates on promoting the checking of each by the others. The task of separation summons forth a "formalist" analysis; it requires formal definitions of some sort to provide the baseline for analysis. The task of checking and balancing is most closely associated with a "functionalist" approach; it requires an awareness of the need to balance the roles and functions of different institutions in determining their appropriate relations.”