XML DTD

Focus on the ID and IDREF attributes: A valid document needs to have unique values across ID attributes. An IDREF attribute can refer to any existing ID attribute value.

<meal>
  <person name="Alice"/>
  <person name="Bob"/>
  <person name="Carol"/>
  <person name="Dave"/>
  <food name="salad"/>
  <food name="turkey"/>
  <food name="sandwich"/>
  <eats diner="Alice" dish="turkey"/>
  <eats diner="Bob" dish="salad"/>
  <eats diner="turkey" dish="Dave"/>
</meal>

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