<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:complexType name="article_type">
        <xs:sequence>
            <xs:element name="title" type="xs:string"/>
            <xs:element name="author" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="date" type="xs:string"/>
        <xs:attribute name="level" type="xs:string"/>
    </xs:complexType>
    
    <xs:element name="article" type="article_type"/>
    
    <xs:complexType name="journal_type">
        <xs:sequence>
            <xs:element ref="article" minOccurs="0" 
                                      maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="title" type="xs:string"/>
        <xs:attribute name="publisher" type="xs:string"/>
    </xs:complexType>

    <xs:element name="journal" type="journal_type"/>
    
    <xs:complexType name="catalog_type">
        <xs:sequence>
            <xs:element ref="journal" minOccurs="0" 
                                      maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
    
    <xs:element name="catalog" type="catalog_type"/>
</xs:schema>

