nom::flat_map

macro_rules! flat_map {
    ($i:expr, $submac:ident!( $($args:tt)* ), $submac2:ident!( $($args2:tt)* )) => { ... };
    ($i:expr, $submac:ident!( $($args:tt)* ), $g:expr) => { ... };
    ($i:expr, $f:expr, $g:expr) => { ... };
    ($i:expr, $f:expr, $submac:ident!( $($args:tt)* )) => { ... };
}

flat_map!(R -> IResult<R,S>, S -> IResult<S,T>) => R -> IResult<R, T>

combines a parser R -> IResult and a parser S -> IResult to return another parser R -> IResult