XML in a Nutshell, 3rd Edition

Child Elements

Another simple content specification is one that says the element must have exactly one child of a given type. In this case, the content specification consists of the name of the child element inside parentheses. For example, this declaration says that a fax element must contain exactly one phone_number element:

<!ELEMENT fax (phone_number)>

A fax element may not contain anything else except the phone_number element, and it may not contain more or less than one of those.