code: public boolean add(E o) {
      	if(o!=null && !contains(o)){
      		return super.add(o);
      	}
      	return false;
      }
docs: Appends the specified element to the end of this list.
  
  @param o element to be appended to this list.
  @return <tt>true</tt> (as per the general contract of Collection.add).