Function select_first 

pub fn select_first<'rule, T, M, R>(
    _: &T,
    rs: &mut dyn Iterator<Item = (&'rule R, Update<T, M>)>,
) -> Option<(&'rule R, Update<T, M>)>
where
    T: Uniplate,
    R: Rule<T, M>,

Returns the first available (Rule, Update) if there is one, otherwise returns None.

This is a good default selection strategy, especially when you expect only one possible rule to apply to any one term.

See the module-level documentation for more information.