Escape from dependency hell
 changing package versions transitively
You want a package which requires a more recent version of gtkLibs? You can't just pass a newer gtkLib version because your package depends on other packages which also depend on gtkLibs? Use pkgsFunDeepOverride to replace a package (eg selecting a newer version) within the dependency graph: mailinglist thread
I'm sure we'll hit cases where this doesn't help either without further adjustments. But for now it does.