To account for the fact that the source voltage is known, we consider both node a and node b as
part of one larger node represented by the shaded ellipse shown in Figure 4.3-2. We require a larger
node because Va and Vb are dependent. This larger node is often called a supernode or a generalized
node. KCL says that the algebraic sum of the currents entering a supemode is zero. That means that we
apply KCL to a supemode in the same way that we apply KCL to a node.