fn spaced_with<F>(self, spacer: F) -> Spacing<Self, F>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Option<Self::Item>,
    {
        Spacing {
            it: self,
            state: SpacingState::NotStarted,
            spacer,
        }
    }
Space a sequence of items by sometimes inserting another item.
packages/stylist-macros/src/spacing_iterator.rs