Xcode 4 shortcut for swapping focus between editor and assistant editor

In Xcode, I am big fan of the assitant editor that shows me the corresponding .h or .m for the file that I am editing.

Is there a shortcut that allows me to swap focus between this two windows? I frequently switch between the two and using the mouse every time is annoying.

New answer:

Move Focus To Editor — commandj followed by ←/↑/↓/→ and return

This goes nicely together with commandshiftj which is Reveal in Project Navigator.

Old answer:

Use optioncommand` keyboard shortcut.

It can be remapped in Preferences - Keyboard Bindings - Move Focus To Next Area.

Edit: Removed XVim recommendation.

I made this to help answer another question... Does it help?

@Oneiros: Not quite what the OP was asking for..

I don't know of a 'short' shortcut but there's Cmd-J showing a popup where you can choose what to focus.