nom::map_opt!

macro_rules! map_opt(
  ($i:expr, $submac:ident!( $($args:tt)* ), $submac2:ident!( $($args2:tt)* )) => (
    {
      match $submac!($i, $($args)*) {
        $crate::IResult::Error(e)                            => $crate::IResult::Error(e),
        $crate::IResult::Incomplete($crate::Needed::Unknown) => $crate::IResult::Incomplete($crate::Needed::Unknown),
        $crate::IResult::Incomplete($crate::Needed::Size(i)) => $crate::IResult::Incomplete($crate::Needed::Size(i)),
        $crate::IResult::Done(i, o)                          => match $submac2!(o, $($args2)*) {
          Some(output) => $crate::IResult::Done(i, output),
          None         => $crate::IResult::Error($crate::Err::Position($crate::ErrorCode::MapOpt as u32, $i))
        }
      }
    }
  );
  ($i:expr, $submac:ident!( $($args:tt)* ), $g:expr) => (
    map_opt!($i, $submac!($($args)*), call!($g));
  );
  ($i:expr, $f:expr, $g:expr) => (
    map_opt!($i, call!($f), call!($g));
  );
  ($i:expr, $f:expr, $submac:ident!( $($args:tt)* )) => (
    map_opt!($i, call!($f), $submac!($($args)*));
  );
);

maps a function returning an Option on the output of a parser