ce in which they were written.<p>

    <h4>Visibility rules</h4>

    Obr programs contain only one scope: the global scope.  Hence all
    identifiers must be declared as either parameters to the program
    or in the global declaration section.  Thus the visibility rules
    of Obr are very simple: every applied occurrence of an identifier
    must <em>identify</em> a defining occurrence of that identifier in
    the global scope.  It is illegal to have two defining occurrences
    of the same identifier,