anchor.diff:  /// Variant map.
   type Vmap<'k, 'v when 'k : comparison> =
       private
  -        { Vnode : Vnode<'k, 'v>
  +        { Node : Vnode<'k, 'v>
             EmptyArray : Vnode<'k, 'v> array
             MaxDepth : int }
   
  
positive.code: 
              | Singlet hkv' ->
                  
                  // additional entry; convert Singlet to Multiple
                  let idx = hashToIndex hkv.H dep
                  let idx' = hashToIndex hkv'.H dep
                  if idx <> idx' then
  
negative.code:                 // logically should never hit here
                  failwithumf ()
  
          // clashing
          else
              
              // handle clash cases
  