anchor.diff:  open System.Collections.Concurrent
   open System.Collections.Generic
   open System.ComponentModel.Composition
  +open System.Diagnostics
   open System.IO
   open System.Linq
   open System.Threading
  
positive.code:     [<Sealed>]
      type ErrorHandlingBuilder() = 
          
          member inline __.Bind(v, f) = 
              match v with
              | Choice1Of2(x) -> f x
              | Choice2Of2(s) -> Choice2Of2(s)
          
          member inline __.ReturnFrom v = v
          member inline __.Return v = Choice1Of2(v)
          member inline __.Zero() = Choice1Of2()
          
          member inline __.Combine(a, b) = 
              match a, b with
              | Choice1Of2 a', Choice1Of2 b' -> Choice1Of2 b'
  
negative.code:             | Choice1Of2 a', Choice2Of2 b' -> Choice2Of2 b'
              | Choice2Of2 a', Choice2Of2 b' -> Choice2Of2 a'
          
          [<DebuggerStepThrough>]
          member inline __.Delay(f) = f()
          
          [<DebuggerStepThrough>]
          member inline this.TryFinally(body, compensation) = 
              try 
                  this.ReturnFrom(body())
              finally
                  compensation()
          
          [<DebuggerStepThrough>]
          member inline __.TryWith(expr, handler) = 
              try 
                  expr()
              with ex -> handler ex
          
          [<DebuggerStepThrough>]
          member inline this.For(collection : seq<_>, func) = 
              // The whileLoop operator
              let rec whileLoop pred body = 
  