Function null 

pub fn null<'a, F, O>(parser: F) -> impl FnMut(ParseString<'a>)
where
    F: FnMut(ParseString<'a>) -> Result<(ParseString<'a>, O), Err<ParseError<'a>>>,

Convert output of any parser into ParserNode::Null. Useful for working with alt combinator and error recovery functions.