Function nom::combinator::map_opt

pub fn map_opt<I: Clone, O1, O2, E: ParseError<I>, F, G>(
    first: F, 
    second: G
) -> impl Fn(I) -> IResult<I, O2, E> where
    F: Fn(I) -> IResult<I, O1, E>,
    G: Fn(O1) -> Option<O2>,