modify device flows =
    let match = fst device
        modify = snd device
    in
    filter (/= VoidFlow) $ map (\x -> if match x then modify x else VoidFlow) flows

reachability network src dst =
    let flow = Flow $ Or [And [Eq "Src" $ Val src, Eq "Dst" $ Val dst, Eq "Port" Any]] in
    foldl (\acc device -> modify device acc) [flow] network