-// Additionally, the 'root' node of a call graph represents the "entry point"
-// node of the graph, which has an edge to every external method in the graph.
-// This node has a null method pointer.
+// There is a second external node added for calls that leave this module.
+// Functions have a call edge to the external node iff:
+// 1. The function is external, reflecting the fact that they could call
+// anything without internal linkage or that has its address taken.
+// 2. The function contains an indirect function call.
+//
+// As an extension in the future, there may be multiple nodes with a null
+// function. These will be used when we can prove (through pointer analysis)
+// that an indirect call site can call only a specific set of functions.
+//
+// Because of these properties, the CallGraph captures a conservative superset
+// of all of the caller-callee relationships, which is useful for
+// transformations.
+//
+// The CallGraph class also attempts to figure out what the root of the
+// CallGraph is, which it currently does by looking for a function named 'main'.
+// If no function named 'main' is found, the external node is used as the entry
+// node, reflecting the fact that any function without internal linkage could
+// be called into (which is common for libraries).