code: @Nullable
      public Relationship delivers(@Nonnull Person destination, String description) {
          return delivers(destination, description, null);
      }
docs: Adds a unidirectional relationship between this element and a person.
  
  @param destination the target of the relationship
  @param description a description of the relationship (e.g. "sends e-mail to")
  @return the relationship that has just been created and added to the model