anchor.diff:      ==================
       help               
         - display this help message
  +    quit
  +      - quit the program
       errors
         - get error messagaes reported by last parse
       declarations
  
positive.code:     | Parse of bool
      | Error of string
      | Help
  
    /// Parse 'help' command
    let help = string "help" |> Parser.map (fun _ -> Help)
  
    /// Parse 'declarations' command
    let declarations = string "declarations" |> Parser.map (fun _ -> Declarations)
  
negative.code:     | Help, _ ->
          Console.WriteLine(helpText)
          main state
  
      | Error(msg), _ -> 
          Console.Error.WriteLine(msg)
          main state
  
      | Quit, _ -> 
          exit 0
        
    // Run the application!
    do main(State())
  