anchor.diff:  
       // OPTIMIZATION :   This function is implemented using a mutable HashSet and ResizeArray for maximum speed.
       //                  The use of mutation is safe here because it's entirely local.
  -    
  +
       /// Contains the distinct elements which have been seen in the list so far.
       let itemSet = System.Collections.Generic.HashSet<_> (LanguagePrimitives.FastGenericEqualityComparer)
   
  
positive.code: let foldPairwise (folder : 'State -> 'T -> 'T -> 'State) state list =
      // Preconditions
      checkNonNull "list" list
      
      // OPTIMIZATION : If the list is empty or contains just one element,
      // immediately return the input state.
      match list with
  
negative.code:         let takenElements = Array.zeroCreate count
  
          let mutable list = list
  
          // Take the elements from the list and store them in the array.
          for i = 0 to count - 1 do
              takenElements.[i] <- List.head list
  