anchor.diff:          set.Contains key
   
       //
  +    [<CompiledName("Union")>]
  +    let inline union (set1 : IntSet) (set2 : IntSet) : IntSet =
  +        // Preconditions
  +        checkNonNull "set1" set1
  +        checkNonNull "set2" set2
  +
  +        set1.Union set2
  +
  +    //
       [<CompiledName("OfSeq")>]
       let inline ofSeq source : IntSet =
           // Preconditions are checked by the member.
  
positive.code:     member __.Remove (key : int) : IntSet =
          IntSet (
              PatriciaSet.Remove (uint32 key, trie))
  
      /// The IntSet containing the given element.
      static member Singleton (element : int) : IntSet =
  
negative.code:         set.Contains (int key)
  
      //
      [<CompiledName("Union")>]
      let inline union (set1 : TagSet<'Tag>) (set2 : TagSet<'Tag>) : TagSet<'Tag> =
          // Retype as IntSet
          let set1 : IntSet = retype set1
          let set2 : IntSet = retype set2
  
          // Preconditions
          checkNonNull "set1" set1
          checkNonNull "set2" set2
  
          set1.Union set2
          |> retype
  
      //
      [<CompiledName("OfSeq")>]
      let inline ofSeq source : TagSet<'Tag> =
          // Preconditions are checked by the member.
  