Macro em_option 

macro_rules! em_option {
    ($ktp:ty, $vtp:ty, $($x:ident=>$y:expr),* ) => { ... };
}

Initializes an EnumMap with Option::None for all variants not explicitly specified.

use enum_collections::{em_option, Enumerated, EnumMap};
#[derive(Enumerated)]
enum Letter {
  A,
  B,
}

let enum_map = em_option!(Letter, i32, A => 42);
assert_eq!(Some(42), enum_map[Letter::A]);
assert_eq!(None, enum_map[Letter::B]);