valid :: (HasUnitId ipkg,   PackageFixedDeps ipkg,
          HasUnitId srcpkg, PackageFixedDeps srcpkg)
      => IndependentGoals
      -> PlanIndex ipkg srcpkg iresult ifailure
      -> Bool
valid indepGoals index =
    null $ problems indepGoals index
valid :: (HasUnitId ipkg,   PackageFixedDeps ipkg,
          HasUnitId srcpkg, PackageFixedDeps srcpkg)
      => IndependentGoals
      -> PlanIndex ipkg srcpkg iresult ifailure
      -> Bool
valid indepGoals index =
    null $ problems indepGoals index
valid indepGoals index =
    null $ problems indepGoals index