src/main/java/org/organicdesign/fp/collections/PersistentHashSet.java
public static <E> MutableHashSet<E> emptyMutable(Equator<E> eq) {
        return empty(eq).mutable();
    }
public static <E> MutableHashSet<E> emptyMutable(Equator<E> eq) {
        return empty(eq).mutable();
    }
Works around some type inference limitations of Java 8.