anchor.diff:      /// <returns></returns>
       [<CompiledName("Bind2")>]
       let bind2 (binding : 'T -> 'U -> Choice<'V, 'Error>) value1 value2 =
  +        // Preconditions
  +        checkNonNull "value1" value1
  +        checkNonNull "value2" value2
  +
           match value1, value2 with
           | Choice1Of2 result1, Choice1Of2 result2 ->
               binding result1 result2
  
positive.code:     /// <returns></returns>
      [<CompiledName("ToOption")>]
      let toOption (value : Choice<'T, 'Error>) : 'T option =
          match value with
          | Choice1Of2 result ->
              Some result
  
negative.code:     /// <returns></returns>
      [<CompiledName("FoldBack")>]
      let foldBack (folder : 'T -> 'State -> 'State) (value : Choice<'T, 'Error>) (state : 'State) : 'State =
          // Preconditions
          checkNonNull "value" value
  
          match value with
          | Choice1Of2 result ->
              folder result state
  