code: private V replaceValue(Node<V> node, V newValue) {
          // Note: a node is terminal if it already has a value
  	if (node.isTerminal()) {
  	    V old = node.value;
  	    node.value = newValue;
  	    return old;
  	}
  	// the node wasn't already a terminal node (i.e. this char sequence is a
  	// substring of an existing sequence), assign it a value
  	else {
  	    node.value = newValue;
  	    size++;
  	    return null; // no old value
  	}
      }
docs: Replaces the value of the provided {@code Node} and returns the old value
  or {@code null} if one was not set.
  
  @param node
  @param newValue
  
  @return