checkPackageDbStack :: PackageDBStack -> IO ()
checkPackageDbStack (GlobalPackageDB:rest)
  | GlobalPackageDB `notElem` rest = return ()
checkPackageDbStack :: PackageDBStack -> IO ()
checkPackageDbStack (GlobalPackageDB:rest)
  | GlobalPackageDB `notElem` rest = return ()
checkPackageDbStack (GlobalPackageDB:rest)
  | GlobalPackageDB `notElem` rest = return ()