in src/main/java/com/twitter/whiskey/util/UniqueMultiMap.java [64:76]
public K removeValue(V value) {
int sentinel = mutations;
K key = inverse.remove(value);
if (key == null) return null;
Deque<V> values = map.get(key);
if (!values.remove(value)) throw new IllegalStateException();
size--;
if (values.isEmpty()) map.remove(key);
if (sentinel != mutations++) throw new ConcurrentModificationException();
return key;
}