anchor.diff:                                   (collect-function #'cons)
                                    (children-function #'children))
     "Map the FUNCTION over the TREE.
  -If TREE is a `htree:history-tree', start from it's root.
  +If TREE is a `htree:history-tree', start from its root.
   If TREE is a `htree:node', start from it.
   Include results of applying FUNCTION over ROOT if INCLUDE-ROOT is
   non-nil.
  
positive.code: 
  (export 'all-current-branch-nodes)
  (defmethod all-current-branch-nodes ((history history-tree))
    "Return a list of all nodes that belong to the branch the current owner node is on."
    (let ((root (root (current-owner-node history))))
      (cons root (all-children (root (current-owner-node history))))))
  
  
negative.code: (declaim (ftype (function (owner node) boolean) disown))
  (defun disown (owner node)
    "Remove binding between OWNER and NODE.
  Return true if NODE was owned by OWNER, nil otherwise."
    (remhash owner (bindings node)))
  
  (defun entry-equal-p (a b)
  