error[E0499]: cannot borrow `hmap` as mutable more than once at a time
  --> src/main.rs:16:55
   |
16 |             attendees.push(set_if_needed_and_get(&mut hmap, m.to_string()));
   |                                                       ^^^^ mutable borrow starts here in previous iteration of loop
...
19 | }
   | - mutable borrow ends here

error: aborting due to previous error