Macro iff::iff

macro_rules! iff {
    (let $pat:pat = $expr:expr, $($tt:tt)+) => { ... };
    (let $pat:pat = $expr:expr => $block:block) => { ... };
    ($expr:expr, $($tt:tt)+) => { ... };
    ($expr:expr => $block:block) => { ... };
}