<?xml version="1.0"?>

<!-- required data field for BOOKLIST -->
<!ELEMENT BOOKLIST (BOOK+)>

<!-- required data field for each BOOK -->
<!ELEMENT BOOK (TITLE, AUTHOR+, ISBN, (LISTPRICE|PRICE), (REQUIRED|RECOMMENDED), OUTOFPRINT?) >
    <!-- (mandatory fields) -->
    <!-- | menas or -> (PRICE|LISTPRICE) -->
    <!-- + menas one or more -> (AUTHOR+) -->
    <!-- ? menas optional -> (LISTPRICE?) -->
    <!-- * menas zero or more -->


<!-- specification of data types for each data field listed above -->
<!ELEMENT TITLE (#PCDATA)>
    <!-- (#PCDATA) means any character that is parsable, i.e. &lt; &gt; and &amp; are encoded!! -->
<!ELEMENT AUTHOR (#PCDATA)>
<!ELEMENT ISBN (#PCDATA)>
<!ELEMENT LISTPRICE (#PCDATA)>
<!ELEMENT PRICE (#PCDATA)>
<!ELEMENT REQUIRED EMPTY>
<!ELEMENT RECOMMENDED EMPTY>
<!ELEMENT OUTOFPRINT EMPTY>
    <!-- EMPTY means just a tag with no information -->