nom::map_opt!

macro_rules! map_opt (
  ($i:expr, $submac:ident!( $($args:tt)* ), $g:expr) => (
    map_opt_impl!($i, $submac!($($args)*), call!($g));
  );
  ($i:expr, $submac:ident!( $($args:tt)* ), $submac2:ident!( $($args2:tt)* )) => (
    map_opt_impl!($i, $submac!($($args)*), $submac2!($($args2)*));
  );
  ($i:expr, $f:expr, $g:expr) => (
    map_opt_impl!($i, call!($f), call!($g));
  );
  ($i:expr, $f:expr, $submac:ident!( $($args:tt)* )) => (
    map_opt_impl!($i, call!($f), $submac!($($args)*));
  );
);

map_res!(I -> IResult<I,O>, O -> Option<P>) => I -> IResult<I, P> maps a function returning an Option on the output of a parser