Reference element
A reference element just consists of a property (or variable) name: Reference elements can be used at any position in a path.
persons.children.children
Usually, reference elements will not change selection and do refer to the currently selected instance in the path. Especially in OSI expressions, this is used to selects an instance in a collection and referring to it in the next statement(s).
The application is responsible to select property instances in reference elements, because otherwise, the path returns nothing.
As long as the path uses reference elements, those will refer to the preselected instances. When the path, however, contains an iteration or selector element, subsequent reference elements do not have the same selection and are known in the context of the path, only. Thus,
persons.children.children
and
persons.children().children
refer to the same selection in person, but children () usually has a different selection from children and so do all subsequent reference elements.
person.Get(0); // selects the first person in the collection
Message(person.name); // prints the name of the selected person
if ( person.children.Next ) // selects the next child for the selected pwerson
Message(person.children.name); // prints the name of the selected child