I've started writing really complicated style files for my TeX documents. Here are a few useful hints that I've come across and feel that I need to write down else I'll forget them.

---

Expandafter

The rules for expanding \expandafter can be fairly complicated. Here's an example of a command called on two macros with the macros expanded before the call:

\expandafter
  \expandafter
  \expandafter
  \command\expandafter
            \expandafter
            \expandafter
              {\expandafter
               \argone
               \argtwo}

Here's the same where the command takes the two arguments separately

\expandafter
  \expandafter
  \expandafter
  \command\expandafter
            \expandafter
            \expandafter
              {\expandafter
                \argone
                \expandafter
              }\expandafter
              {\argtwo}

Phew!

A useful trick for figuring out \expandafters is to use the delete-word and forward-word shortcuts in emacs. More-or-less, taking a line without extraneous spaces and doing delete-word + forward-word on each expandafter leaves you with what TeX does. Repeated doing of this can give you insight into what TeX does when it encounters a complicated string such as the above.