Add comment for operator associativity

src/assignments/assignment04/parser.rs

@@ -14,6 +14,12 @@ mod inner {
 }
 
 /// Parses command.
+///
+/// ## Operator Associativty
+///
+/// For associativity of each operator, please follow [here](https://docs.rs/pest/latest/pest/prec_climber/struct.PrecClimber.html#examples).
+///
+/// e.g. `1+2+3` should be parsed into `(1+2)+3`, not `1+(2+3)` because the associativity of plus("add" in our hw) operator is `Left`.
 pub fn parse_command(line: &str) -> Result<Command> {
     todo!("fill here")
 }