anchor.diff:  
       /// A garbage-collected keyed array.
       /// TODO: once this is well-tested, let's consider moving into Prime.
  -    type [<NoEquality; NoComparison>] KeyedArray<'k, 'v> =
  +    type [<NoEquality; NoComparison>] KeyedArray<'k, 'v when 'k : equality> =
           private
               { Keys_ : SortedDictionary<int, 'k> // sorted so that compacting does not change order
                 Indices_ : Dictionary<'k, int>
  
positive.code: 
  /// A garbage-collected keyed array.
  /// TODO: once this is well-tested, let's consider moving into Prime.
  type KeyedArray<'k, 'v when 'k : comparison> = KeyedArray.KeyedArray<'k, 'v>
negative.code:         &karr.[key]
  
      /// Make a KeyedArray with the given compaction threshold and initial capaticy.
      let make<'k, 'v when 'k : equality> threshold capacity : KeyedArray<'k, 'v> =
          { Keys_ = SortedDictionary<int, 'k> ()
            Indices_ = Dictionary<'k, int> ()
            Values_ = Array.zeroCreate capacity
  