Programmers routinely document their source code. These comments usually reside at the line level or the function level. However, there are many layers of complexity between the line level and the function level. For example, a function may contain several loops (possibly nested). Each loop may be thought of as doing a “chunk” of work that could be documented.
Wouldn't it be nice to have a code editor which somehow showed the abstract syntax of the program and allowed annotations to be added at whatever level of abstraction was desired? The resulting nested layers of annotation could be used for many purposes, for example, introducing a new programmer to the program in increasing levels of detail.