code: public boolean add(Visitor visitor) {
  
      Assert.notNull(visitor, "The Visitor to add to this composite cannot be null");
  
      return visitor != this && !contains(visitor) && visitors.add(visitor);
    }
docs: Adds the specified Visitor to the composition (collection) of Visitors.  The Visitor will be added if and only if
  the Visitor is not null and is not already a member of this composition.
  
  @param visitor the Visitor object to add to this composition.
  @return a boolean value indicating whether the Visitor was added successfully or not.
  @throws NullPointerException if the Visitor is null.
  @see #contains(org.cp.elements.lang.Visitor)