[Marco Maggi] - In this page I present the code for an experimental tree
stucture. I  don't like it at  all because '''I  Believe''' that complex
data structures have  to be implemented at the C level.  But, I needed a
tree structure in my Wiki pages so...

It's probably incorrect  in some operations, I'll try to  test it when I
find time and will to do it.
----
 proc tree_set_root { treevar node } {
    upvar       $treevar tree
    array set tree [list root $node $node:dad {} $node:cld {}]
 }
 proc tree_get_root { treevar } {
    upvar       $treevar tree
    return $tree(root)
 }
 proc tree_isroot { treevar node } {
    upvar       $treevar tree
    return [expr {([string equal $tree(root) $node])? 1 : 0}]
 }
 proc tree_get_children { treevar node } {
    upvar       $treevar tree
    return $tree($node:cld)
 }
 proc tree_get_father { treevar node } {
    upvar       $treevar tree
    return $tree($node:dad)
 }
 proc tree_exists { treevar node} {
    upvar       $treevar tree
    return [info exists tree($node:cld)]
 }
 proc tree_add_children { treevar node children } {
    upvar       $treevar tree

    foreach child $children {
        if { [info exists tree($child:dad)] } {
            tree_remove_children tree($child:dad) $child
        } else {
            set tree($child:cld) {}
        }
        lappend tree($node:cld) $child
        set tree($child:dad) $node
    }
    return
 }
 proc tree_remove { treevar node } {
    upvar       $treevar tree

    if { [string equal $tree(root) $node] } {
        set tree(root) {}
        foreach child $tree($node:cld) {
            tree_remove tree $child
        }
    } else {
        set dad $tree($node:dad)
        tree_remove_child tree $dad $node
    }
    unset tree($node:dad) tree($node:cld)
 }
 proc tree_remove_child { treevar node child } {
    upvar       $treevar tree

    set idx [lsearch $tree($node:cld) $node]
    set tree($node:cld) [lreplace $tree($node:cld) $idx $idx]
    set tree($child:dad) {}
    return
 }
 proc tree_path { treevar node } {
    upvar       $treevar tree

    set path {}
    while { ! [string equal $tree(root) $node] } {
        set path [linsert $path 0 $node]
        set node $tree($node:dad)
    }
    return [linsert $path 0 $node]
 }
----
To test it we can fill it with the following procedure. 
----
 proc fill_tree { treevar } {
    upvar       $treevar tree

    tree_set_root tree 0
    tree_add_children tree 0 { 1 2 3 4 5 6 7 8 9 }

    for {set i 1} {$i < 10} {incr i} {
        for {set j 1} {$j < 10} {incr j} {
            tree_add_children tree $i $i.$j
            for {set k 0} {$k < 10} {incr k} {
                tree_add_children tree $i.$j $i.$j.$k
            }
        }
    }
 }
----
[Keep your struct away from me] - [Category Data Structure]