Function pear::parsers::take_n_if

pub fn take_n_if<I, F>(
    input: &mut Pear<I>, 
    n: usize, 
    cond: F
) -> Result<I::Many, I> where
    I: Input,
    F: FnMut(&I::Token) -> bool, 

Take exactly n tokens, ensuring cond holds on all n.