anchor.diff:                  ]
           go >> P.run
   
  -    /// Configurations for building a tree.
  +    /// Configurations for building a tree.    
       type IBuildTreeConfiguration =
           abstract BuildMap<'K,'V when 'K : comparison> : seq<'K * 'V> -> IRangeMap<'K,'V>
   
  
positive.code:         {
              /// Filter type
              Level : 'L
  
              /// All items
              AllElements : option<'T []>
  
              /// Child nodes
              Children : IRangeMap<'K, Tree<'K,'L,'T>>
  
negative.code:         | Node n -> 
              Node {
                  Level = n.Level
                  Children =  n.Children.Map (fun t -> map f t) 
              }
          | Leaf x    -> 
  