= if ipid_old `elem` preferred
then ( (ipid_new, ShadowedBy ipid_old) : shadowed, pkgmap )
else ( (ipid_old, ShadowedBy ipid_new) : shadowed, pkgmap' )
= if ipid_old `elem` preferred
then ( (ipid_new, ShadowedBy ipid_old) : shadowed, pkgmap )
else ( (ipid_old, ShadowedBy ipid_new) : shadowed, pkgmap' )