digraph lattice {
    Top -> NullableStr -> Str -> Char -> Bottom;
    Top -> Int -> NonNegInt -> Bool -> Bottom;
    Top -> "*instances*" -> Bottom;
    "*instances*" -> None;
    NullableStr -> None;
    Top -> "*PBCs*" -> Bottom;
    "*PBCs*" -> None;
    Top -> "*lists*" -> None -> Bottom;

    "*lists*" [shape=box];
    "*instances*" [shape=box];
    "*PBCs*" [shape=box];
}
