constant B = "B", R = "R"

function balance(sequence t)
    sequence s = match_algebraic({{B,{R,{R,0,0,0},0,0},0,0},
                                  {B,{R,0,0,{R,0,0,0}},0,0},
                                  {B,0,0,{R,{R,0,0,0},0,0}},
                                  {B,0,0,{R,0,0,{R,0,0,0}}}},t)
    if length(s) then
        object {a,x,b,y,c,z,d} = s
        t = {R,{B,a,x,b},y,{B,c,z,d}}
    end if
    return t
end function

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

sequence stuff = shuffle(tagset(10))
object tree = NULL
for i=1 to length(stuff) do
    tree = tree_insert(tree,stuff[i])
end for
visualise_tree(tree)