<?xml version="1.0"?>
<interfaceDefinition extends="system">
  <node name="login">
    <tagNode name="user">
      <helpString>User name</helpString>
      <constraint>
        <regex>[a-z][a-zA-Z0-9]+</regex>
        <errorMessage>User name must start with a letter and consist of letters and digits</errorMessage>
      </constraint>
      <leafNode name="full-name">
        <helpString>User full name</helpString>
      </leafNode>
    </tagNode>
  </node>
</interfaceDefinition>

Add an example of interface definition.
Add an example of interface definition.
