P5: Guidelines for Electronic Text Encoding and Interchange

<rng:define name="data.certainty">
 <rng:choice>
  <rng:value>high</rng:value>
  <rng:value>medium</rng:value>
  <rng:value>low</rng:value>
  <rng:value>unknown</rng:value>
 </rng:choice>
</rng:define>

data.certainty = "high" | "medium" | "low" | "unknown"