src/main/java/org/organicdesign/fp/collections/PersistentHashMap.java
public static <K,V> MutableHashMap<K,V> emptyMutable(Equator<K> e) {
        return PersistentHashMap.<K,V>empty(e).mutable();
    }
public static <K,V> MutableHashMap<K,V> emptyMutable(Equator<K> e) {
        return PersistentHashMap.<K,V>empty(e).mutable();
    }
Works around some type inference limitations of Java 8.