Index: llvm/include/llvm/ADT/GraphTraits.h
===================================================================
--- llvm/include/llvm/ADT/GraphTraits.h
+++ llvm/include/llvm/ADT/GraphTraits.h
@@ -25,6 +25,14 @@
 // GraphTraits - This class should be specialized by different graph types...
 // which is why the default version is empty.
 //
+// GraphTraits is not designed in the most convenient and intuitive way, as it
+// started with simple and limited scenarios (aka BasicBlock), and over time
+// evolved to facncier applications (e.g. CFG and DomTree).
+//
+// One of the common "fancy" uses of GraphTraits is to create a view of a
+// physical graph. The view interprets the physical graph slightly differently
+// without incurring significant memory footprint. Such application is possible
+// by hacking on NodeRef. LoopBodyTraits is a good example.
 template<class GraphType>
 struct GraphTraits {
   // Elements to provide: