anchor.diff:      /// Compute the difference of two PatriciaMaps.
       static member Difference (s, t) : PatriciaSet =
           match s, t with
  -        | (Br (p, m, s0, s1) as s), (Br (q, n, t0, t1) as t) ->
  +        | Br (p, m, s0, s1), Br (q, n, t0, t1) ->
               if m = n then
                   if p <> q then s
                   else
  
positive.code:         | Br (_,_,_,_), (Empty | Lf _) -> 1
          | Lf x, Lf y ->
              if x = y then 0
              else 1  // The maps are disjoint.
  
          | Lf x, Br (p, m, l, r) ->
              if not <| matchPrefix (x, p, m) then 1
              elif zeroBit (x, m) then
                  match PatriciaSet.SubsetCompare (t1, l) with
                  | 1 -> 1
                  | _ -> -1
              else
                  match PatriciaSet.SubsetCompare (t1, r) with
                  | 1 -> 1
                  | _ -> -1
  
          | Lf _, Empty ->
              // The maps are disjoint.
  
negative.code:         match set with
          | Empty ->
              false
          | Lf j ->
              key = j
          | Br (_, m, t0, t1) ->
              PatriciaSet.Contains
  