Besides using the content model of ANY, you can specify that the element you're declaring contains another element by giving the name of that element in parentheses, like this:
<?xml version = "1.0" standalone="yes"?> <!DOCTYPE DOCUMENT [ <!ELEMENT DOCUMENT (CUSTOMER)*> ]> <DOCUMENT> . . . </DOCUMENT>
In this case, I'm indicating that the root element, <DOCUMENT>, can contain any number (including zero) of <CUSTOMER> elements. (The way I specify that the <DOCUMENT> element can contain any number of <CUSTOMER> elements is with the asterisk after the parentheses, and we'll see how that works in a page or two.)
The <DOCUMENT> element can contain any number of <CUSTOMER> elements, so I can now add a <CUSTOMER> element to the ...