anchor.diff:  
               let result =
                   let result, head', stream'' =
  -                    CharDiet.TryDeleteMin stream
  -                    ||> unionImpl input None
  +                    let min, stream' = CharDiet.TryDeleteMin stream
  +                    
  +                    assert (dietInvariant stream')
  +                    assert (intervalsDisjoint stream')
  +                    
  +                    unionImpl input None min stream'
   
                   match head' with
                   | None ->
  
positive.code:                 // one of the nodes must have height > height r + 1
                  if AvlTree.Height ll >= AvlTree.Height lr then
                      AvlTree.Create (
                          ln,
                          ll,
                          AvlTree.Create (n, lr, r))
                  else
                      // balance right: combination
                      match lr with
  
negative.code:         // Preconditions
          //assert (AvlTree.AvlInvariant l)
          //assert (AvlTree.AvlInvariant r)
          assert (AvlTree.HeightDiff (l, r) <= (balanceTolerance + 1u))
  
          let lh = AvlTree.Height l
          let rh = AvlTree.Height r
  