$ sort -f -s -u letters.txt

$ prog [-options] [args...]

Boolean options of the form: -flag, --flag, and /flag. Boolean parameters can have a `+' or `-' appended to explicitly enable or disable the flag (in the same fashion as mcs -debug+). For boolean callbacks, the provided value is non-null for enabled, and null for disabled.
    Value options with a required value (append `=' to the option name) or an optional value (append `:' to the option name). The option value can either be in the current option (--opt=value) or in the following parameter (--opt value). The actual value is provided as the parameter to the callback delegate, unless it's (1) optional and (2) missing, in which case null is passed.
    Bundled parameters which must start with a single `-' and consists of a sequence of (optional) boolean options followed by an (optional) value-using option followed by the options's vlaue. In this manner,
-abc would be a shorthand for -a -b -c, and -cvffile would be shorthand for -c -v -f=file (in the same manner as tar(1)).
    Option processing is disabled when -- is encountered.

bool show_help = false;
List<string> names = new List<string> ();
int repeat = 1;

var p = new OptionSet () {
    {"n|name=","the {NAME} of someone to greet.",
       v => names.Add (v) },
    {"r|repeat=",
      "the number of {TIMES} to repeat the greeting.
" +
         "this must be an integer.",
        (int v) => repeat = v },
    {"v","increase debug message verbosity",
       v => { if (v != null) ++verbosity; } },
    {"h|help", "show this message and exit",
       v => show_help = v != null },
};

List<string> extra;
try {
    extra = p.Parse (args);
}
catch (OptionException e) {
    Console.Write ("greet:");
    Console.WriteLine (e.Message);
    Console.WriteLine ("Try `greet --help' for more information.");
    return;
}