@Override
    public Optional<MemberShape> getMember(String memberName) {
        return Optional.ofNullable(members.get().get(memberName));
    }
Builder used to create a List or Set shape.
@param <B> Concrete builder type.
@param <S> Shape type being created.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/NamedMembersShapeBuilder.java