Given:

• an xml defining elements and groups;
• each element belongs to a group or groups;
• group may belong to another group.

Find:

• groups, a given element directly or inderectly belongs to;
• a function checking whether an element belongs to a group.

Example:

<groups>
  <group name="g1">
    <element ref="e1"/>
    <element ref="e2"/>
    <element ref="e3"/>
    <group ref="g2"/>
  </group>
  <group name="g2">
    <element ref="e5"/>
  </group>
  <group name="g3">
    <element ref="e1"/>
    <element ref="e4"/>
  </group>
</groups>

There are several solutions depending on aggresiveness of optimization. A moderate one is done through the xsl:key. All this reminds recursive common table expressions in SQL.

Anyone?