anchor.diff:      var y
   
       // reset to null when option not specified
  -    @Option(names = ["-x"], defaultValue = Option.NULL)
  +    @Option(names = ["-x"], defaultValue = Option.NULL_VALUE)
       fun setX(x: String) { this.x = x }
   
       // retains value of previous parseArgs invocation when option not specified
  
positive.code: ${bundle:a:-${env:b:-${sys:c:-X}}}
  ```
  
  The above variable is expanded as follows. First, try to find key `a` in the command's resource bundle. If `a` is not found in the resource bundle, get the value of environment variable `b`. If no environment variable `b` exists, get the value of system property `c`. Finally, no system property `c` exists, the value of the expression becomes `X`.
  
  === Escaping Variables
  Sometimes you want to show a string like `"${VAR}"` in a description.
  
negative.code: 
  Starting from v2.1.0, picocli supports "argument files" or "@-files".
  Argument files are files that themselves contain arguments to the command.
  When picocli encounters an argument beginning with the character `@`,
  it expands the contents of that file into the argument list.
  
  An argument file can include options and positional parameters in any combination.
  