Use tabs instead of spaces or the wrath of retab will find you.
retab FILENAME [NEWFILE]
If no new file is provided, it will be modified in place.
Retab will ignore any line beginning with a tab, so snippets can be pasted into an already tabbed document and retab can be run on them.
Note - however, that retab uses the first line it finds with a space as the basis, so any existing space-prepended lines will be tabbed in and used.