specifies that element classroom must contain exactly one teacher element followed
by exactly one student element. The content specification can contain any number of
items in sequence.
which specifies that element dessert must contain either one iceCream element or one
pastry element, but not both. The content specification may contain any number of pipe
character-separated choices.
An element’s frequency (i.e., number of occurrences) is specified by using either the
plus sign (+), asterisk (*) or question mark (?) occurrence indicator (Fig. 6.4).
A plus sign indicates one or more occurrences. For example,