The COD method's edge addition strategy and its biological motivation builds on a concept of weak siblings. We call a pair of nodes weak siblings if and only if they are connected to the exactly the same set of neighbors, but are not connected to each other. In terms of protein interaction networks, weak siblings are proteins which interact with the same set of proteins but do not interact with each other. In particular, proteins that can substitute each other in a protein interaction network may have this property. Similarly, a pair of proteins that belong to the same complex but are not connected due to missing data or an experimental error will be represented as weak siblings. Since the weak siblings relationship suggests functional similarity, the COD method takes a first step towards delineation of functional groups by connecting every pair of weak siblings by an edge. As this modification may also eliminate some of the chordless cycles of length four (squares) in the graph, functional group delineation happens simultaneously with transformation of the protein interaction graph into a chordal graph.