code: public void removeMapping(K key, V value) {
      if (treatCollectionsAsImmutable) {
        Collection<V> c = map.get(key);
        if (c != null) {
          Collection<V> newC = cf.newCollection();
          newC.addAll(c);
          newC.remove(value);
          map.put(key, newC);
        }
  
      } else {
        Collection<V> c = get(key);
        c.remove(value);
      }
    }
docs: Removes the value from the Collection mapped to by this key, leaving the
  rest of the collection intact.
  
  @param key
  the key to the Collection to remove the value from
  @param value
  the value to remove