code: public boolean remove(K key, V value) {
          Set<V> set = storage.get(key);
          if (set != null) {
              final boolean removed = set.remove(value);
              if (removed && set.isEmpty()) {
                  storage.remove(key);
              }
              return removed;
          }
          return false;
      }
docs: Removes {@code key}-{@code value} pair if it exists.
  
  @return {@code true} if pair was removed, {@code false} otherwise