Function lip::problem

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

Indicate that a parser has reached a dead end.

“Everything was going fine until I ran into this problem.”