test4 =
    runST $
    do
    t <- list_dubst [3,6,2,5,1]
    -- dubst_member 5 t
    dubst_member 4 t

-- convert from DU tree to high level tree
test4 =
    runST $
    do
    t <- list_dubst [3,6,2,5,1]
    -- dubst_member 5 t
    dubst_member 4 t

-- convert from DU tree to high level tree
test4 =
    runST $
    do
    t <- list_dubst [3,6,2,5,1]
    -- dubst_member 5 t
    dubst_member 4 t

-- convert from DU tree to high level tree