... It is possible to change the meaning of keywords (to `overload' them), but it can be confusing, primarily because your new commands may not behave the same way that this manual claims. ... If you said box in your macro, then overloaded the keyword, you'd have a macro that called itself. ... Another example is that (when overloaded) lines saves the line numbers used, so that you can write a macro to print the top 10 lines of a file (it's called head ). ...