anchor.diff:  (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, false otherwise."
  +Return true if NODE was owned by OWNER, nil otherwise."
     (remhash owner (bindings node)))
   
   (defun entry-equal-p (a b)
  
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:                                  (collect-function #'cons)
                                   (children-function #'children))
    "Map the FUNCTION over the TREE.
  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.
  