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 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
  
negative.code: let choosei (chooser : int -> 'T -> 'U option) list : 'U list =
      // Preconditions
      checkNonNull "list" list
  
      let chooser = FSharpFunc<_,_,_>.Adapt chooser
  
      let rec choosei chosen index list =
  