curl -XGET --user username:apikey "https://www.bibsonomy.org/api/concepts"

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<bibsonomy stat="ok">
  <tags start="0" end="2">
    <tag name="concept1" globalcount="83" usercount="15">
      <subTags start="0" end="2">
        <tag usercount="0" name="subtag1" globalcount="0"/>
        <tag usercount="0" name="subtag2" globalcount="0"/>
      </subTags>
    </tag>
    <tag name="concept2" globalcount="53" usercount="45">
      <subTags start="0" end="2">
        <tag usercount="0" name="another_subtag1" globalcount="0"/>
        <tag usercount="0" name="another_subtag2" globalcount="0"/>
      </subTags>
    </tag>
    <tag name="concept3" globalcount="25" usercount="19">
      <subTags start="0" end="3">
        <tag usercount="0" name="yet_another_subtag1" globalcount="0"/>
        <tag usercount="0" name="yet_another_subtag2" globalcount="0"/>
        <tag usercount="0" name="yet_another_subtag3" globalcount="0"/>        
      </subTags>
    </tag>
  </tags>
</bibsonomy>

<!--
  this type encapsulated a list of tags
-->
<xsd:complexType name="TagsType">
  <xsd:sequence>
    <xsd:element name="tag" type="TagType" minOccurs="0" maxOccurs="unbounded"/>
  </xsd:sequence>
  <xsd:attribute name="start" type="xsd:positiveInteger" use="required"/>
  <xsd:attribute name="end" type="xsd:positiveInteger" use="required"/>
  <xsd:attribute name="next" type="xsd:anyURI"/>
</xsd:complexType>

<xsd:complexType name="TagType">
  <xsd:sequence>
    <xsd:element maxOccurs="unbounded" minOccurs="0" name="subTags" type="TagsType"/>
    <xsd:element maxOccurs="unbounded" minOccurs="0" name="superTags" type="TagsType" />
  </xsd:sequence>
  <xsd:attribute name="name" type="xsd:string" use="required"/>
  <xsd:attribute name="globalcount" type="xsd:positiveInteger"/>
  <xsd:attribute name="usercount" type="xsd:positiveInteger"/>
</xsd:complexType>