Function lip::problem

pub fn problem<'a, F1: 'a, F2: 'a, A: 'a, S: Clone + 'a>(
    message: String, 
    from: F1, 
    to: F2
) -> BoxedParser<'a, A, S> where
    F1: Fn(Location) -> Location,
    F2: Fn(Location) -> Location, 

Indicate that a parser has reached a dead end.

"Everything was going fine until I ran into this problem."