anchor.diff:  javac  ..\src\seedu\addressbook\Addressbook.java -d ..\bin
   IF ERRORLEVEL 1 (
       echo ********** BUILD FAILURE ********** 
  +    REM return to previous directory
  +    popd
       exit /b 1
   )
   REM no error here, errorlevel == 0
  
positive.code: 
  REM compare the output to the expected output
  FC actual.txt expected.txt
  
negative.code: @ECHO OFF
  
  REM change current directory to the location of this script
  pushd %~dp0
  
  REM create bin directory if it doesn't exist
  if not exist ..\bin mkdir ..\bin
  