01_FishTank.java

public class FishTank {
    public List<Map<String, Set<@NotNull String>>> getTagsOfFishesOfTheWeek() { ... }
}

02_Api.java

newMapping
    .type( FishTank.class )
        .property( "tagsOfFishesOfTheWeek", METHOD )
            .typeArgument( 0, 1, 0 )
                .constraint( new NotNullDef() );