Add comments to some examples

Author: qryxip

examples/abc073-d.rs

@@ -13,7 +13,7 @@ use std::ops::Sub;
 
 fn main() {
     input! {
-        _n: usize,
+        _n: usize, // `proconio` does not accept `_` as a variable name until v0.3.5.
         m: usize,
         r: usize,
         rs: [NodeIndex1<u32>; r],

examples/abc118-b-proconio.rs

@@ -8,7 +8,7 @@ use std::ops::{BitAnd, BitOr};
 fn main() {
     input! {
         n: usize,
-        _m: usize,
+        _m: usize, // `proconio` does not accept `_` as a variable name until v0.3.5.
         a: [[Usize1]; n],
     }
 

examples/abc129-f.rs

@@ -16,7 +16,7 @@ fn main() {
         l: u64,
         a: u64,
         b: u64,
-        _m: Mod,
+        _m: Mod, // `proconio` does not accept `_` as a variable name until v0.3.5.
     }
 
     let count = |d| -> _ {