Thanks, I hate it.

I imagined what an Elasticsearch query style Python XML syntax would look like.You must share my pain.

<fun>
  <name>sum</name>
  <in>
    <int>
      <name>foo</name>
    </int>
    <int>
      <name>bar</name>
      <default_value>
        <int>0</int>
      </default_value>
    </int>
  </in>
  <out>
    <int>
      <calculation>
        <numerical_operation>
          <operator_plus>
            <operand>
              <var>foo</var>
            </operand>
            <operand>
              <var>bar</var>
            </operand>
          </operator_plus>
        </numerical_operation>
      </calculation>
    </int>
  </out>
</fun>