evalHist
  :: (TileRef -> History TileRef -> Maybe (TileRef, History TileRef))
  -> World
  -> TileQueryResult
evalHist f wo = case f (wFocus wo) (wHistory wo) of
  Nothing  -> []
  Just (tr, h) -> [(tr, Just h)]
evalHist
  :: (TileRef -> History TileRef -> Maybe (TileRef, History TileRef))
  -> World
  -> TileQueryResult
evalHist f wo = case f (wFocus wo) (wHistory wo) of
  Nothing  -> []
  Just (tr, h) -> [(tr, Just h)]
evalHist f wo = case f (wFocus wo) (wHistory wo) of
  Nothing  -> []
  Just (tr, h) -> [(tr, Just h)]