anchor.diff:              // We will then either return or expand the buffer again.
               nRead <> 0 && tryExpandPastIndex startingIndex idx
       override _.TryExpandPastIndex(startingIndex, idx) =
  +        checkDisposed()
           tryExpandPastIndex startingIndex idx
       override _.LengthSoFar =
           checkDisposed()
  
positive.code: open Operators.Checked
  #endif
  open System
  open System.Collections.Generic
  open System.Diagnostics
  open System.IO
  
negative.code:                     bufferFirstCharacterIndex <- startingIndex
                  else
                      // Otherwise we make the buffer larger.
                      growBuffer()
              let bufferContentLength = getBufferContentLength()
              // It's now time to read more characters.
              let nRead = reader.Read(buffer, bufferContentLength, buffer.Length - bufferContentLength)
  