type Lefts<L, R, Iter> = FilterMap<'static, Either<L, R>, L, Iter>;
An iterator yielding the Left values of its source
Prefix searches with a type followed by a colon (e.g.
fn:) to restrict the search to a given type.
Accepted types are: fn, mod,
struct (or str), enum,
trait, typedef (or
tdef).