code: public LinkedMultiValueMap<K, V> deepCopy() {
          LinkedMultiValueMap<K, V> copy = new LinkedMultiValueMap<>(this.size());
          this.forEach((key, value) -> copy.put(key, new LinkedList<>(value)));
          return copy;
      }
docs: Create a deep copy of this Map.
  
  @return a copy of this Map, including a copy of each value-holding List entry
  @see #clone()