anchor.diff:      | Parse of bool
       | Error of string
       | Help
  +    | Quit
   
     /// Parse 'help' command
     let help = string "help" |> Parser.map (fun _ -> Help)
  +
  +  /// Parse 'quit' command
  +  let quit = string "quit" |> Parser.map (fun _ -> Quit)
   
     /// Parse 'declarations' command
     let declarations = string "declarations" |> Parser.map (fun _ -> Declarations)
  
positive.code:     | Help, _ ->
          Console.WriteLine(helpText)
          main state
      | Error(msg), _ -> 
          Console.Error.WriteLine(msg)
          main state
  
    // Run the application!
    do main(State())
  
negative.code:     ==================
      help               
        - display this help message
      quit
        - quit the program
      errors
        - get error messagaes reported by last parse
      declarations
  