final void add(Iterable<? extends Map.Entry<? extends IN_NAME, String>> entries) {
        if (entries == this) {
            throw new IllegalArgumentException("can't add to itself.");
        }

        if (!addFast(entries)) {
            addSlow(entries);
        }
    }
Validates the given user-specified {@code value}.
core/src/main/java/com/linecorp/armeria/common/StringMultimap.java