mm(1): improves comments

src/mm.rs

@@ -47,7 +47,7 @@ fn main() -> ExitCode {
 	let mut u = false; /* unbuffer i/o */
 	let mut ins = Vec::new(); /* initial input file path vector */
 	let mut outs = Vec::new(); /* initial output file path vector */
-	let mut mode: Option<ArgMode> = None; /* mode for positional arguments */
+	let mut mode: Option<ArgMode> = None; /* mode set by last-used option */
 	let mut optind = 0;
 
 	while let Some(opt) = argv.getopt("aei:o:tu") {
@@ -59,30 +59,36 @@ fn main() -> ExitCode {
 			Ok("i") => { /* add inputs */
 				let input = opt.arg().unwrap();
 				ins.push(input);
-				mode = Some(In);
+				mode = Some(In); /* latest argument == -i */
 			},
 			Ok("o") => { /* add output */
 				let output = opt.arg().unwrap(); 
 				outs.push(output);
-				mode = Some(Out);
+				mode = Some(Out); /* latest argument == -o */
 			},
 			Err(_) | Ok(_) => {
 				eprintln!("{}", usage);
 				return ExitCode::from(EX_USAGE as u8);
 			},
 		};
+
 		optind = opt.ind();
 	}
 
 	let remaining = argv.iter().skip(optind);
 
+	/* check the last flag specified */
 	if let Some(m) = mode {
 		for arg in remaining {
+			/* move the subsequent arguments to the list of inputs or outputs */
 			match m {
 				In => ins.push(arg.to_string()),
 				Out => outs.push(arg.to_string()),
 			};
 		}
+	} else {
+		eprintln!("{}", usage);
+		return ExitCode::from(EX_USAGE as u8);
 	}
 
 	/* use stdin if no inputs are specified */