Function separate 

pub const fn separate<'f, 's, Item, F, W>(
    sep: &'s str,
    t: F,
) -> impl FnOnce(W) -> Result + 's
where
    Item: FnMut(&mut W) -> Result,
    F: FnMut() -> Option<Item> + 's,
    W: Write,

Separates the items yielded by iterating the provided function