Lexer can now tokenize

minus
decrement
return direction
separator

file/error.go

@@ -59,6 +59,7 @@ func (err Error) Error () (formattedMessage string) {
 		// print an arrow with a tail spanning the width of the mistake
 		columnCountdown := err.Location.column
 		for columnCountdown > 1 {
+			// TODO: for tabs, print out a teb instead.
 			formattedMessage += " "
 			columnCountdown --
 		}