final <V extends CONTAINER> V updateParent(V parent) {
        this.parent = requireNonNull(parent, "parent");
        return parent;
    }
Skeletal builder implementation of {@link StringMultimap} and/or its subtypes.

@param <IN_NAME> the type of the user-specified names, which may be more permissive than {@link NAME}
@param <NAME> the actual type of the names
@param <CONTAINER> the type of the container object
@param <SELF> the type of {@code this}
core/src/main/java/com/linecorp/armeria/common/StringMultimapBuilder.java