Red black tree sort/Phix

Since I was invited to and it was about a zillion times easier, I simply added delete routines to the Algebraic data types#Phix task.
Unchanged code omitted for clarity, either copy in the first hundred lines or so from the other task, or use the runnable version in the distro.

-- demo\rosetta\Red_Black_Tree.exw
with javascript_semantics

enum CLR, LEFT, DATA, RIGHT

function ins(object tree, object leaf)
    if tree=NULL then
        tree = {R,NULL,leaf,NULL}
    else
        object {c,l,k,r} = tree
        if leaf!=k then
            if leaf<k then l = ins(l,leaf)
                      else r = ins(r,leaf)
            end if
            tree = balance({c,l,k,r})
        end if
    end if
    return tree
end function

function tree_insert(object tree, leaf)
    tree = ins(tree,leaf)
    tree[1] = B
    return tree
end function

object lm
function leftmost(object tree)
    -- set lm and return tree with that removed
    object l = tree[LEFT]
    if l=NULL then
        lm = tree[DATA]
        tree = tree[RIGHT]
    else
        tree[LEFT] = NULL -- (kill refcount)
        l = leftmost(l)
        tree[LEFT] = l
    end if
    if tree!=NULL then
        tree = balance(tree)
    end if
    return tree
end function

function del(object tree, object leaf)
    if tree!=NULL then
        object {c,l,k,r} = tree
        tree = NULL
        if leaf=k then
            if l=NULL then
                tree = r
            elsif r=NULL then
                tree = l
            else
                r = leftmost(r)
                k = lm
                tree = {c,l,k,r}
            end if
        else
            if leaf<k then
                l = del(l,leaf)
            else
                r = del(r,leaf)
            end if
            tree = {c,l,k,r}
        end if
        if tree!=NULL then
            tree = balance(tree)
        end if
    end if
    return tree
end function

function tree_delete(object tree, leaf)
    tree = del(tree,leaf)
    tree[1] = B
    return tree
end function

procedure main()
    sequence stuff = shuffle(tagset(30))
    object tree = NULL
    for i=1 to length(stuff) do
        tree = tree_insert(tree,stuff[i])
    end for
    stuff = shuffle(stuff)[1..15]
    for i=1 to length(stuff) do
        tree = tree_delete(tree,stuff[i])
    end for
    visualise_tree(tree)
end procedure
main()

Output:

  ┌B3
  │└B4
  │ └R5
 ┌B6
 ││┌B9
 │└R10
 │ └B11
─B15
 │┌B17
 └B19
  │ ┌B20
  │┌B22
  └R24
   └B27
    └B30