Typically HTTP headers work like a `Map<String, String>`: each field has one value or none. But some headers permit multiple values, like Guava's [Multimap](https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Multimap.html). For example, it's legal and common for an HTTP response to supply multiple `Vary` headers. OkHttp's APIs attempt to make both cases comfortable.