xml quicksort

<xml>
  <defun name="qsort">
    <args>
      <required-arg name="lis" />
      <optional-args>
        <optional-arg name="less" default="nil"/>
        <optional-arg name="greater" default="nil"/>
      </optional-args>
    </args>
    <cond>
      <cond-clause>
        <cond-check>
          <null-check>
            <lis />
          </null-check>
        </cond-check>
        <cond-expr>
          <nil />
        </cond-expr>
      </cond-clause>
      <cond-clause>
        <cond-check>
          <null-check>
            <cdr>
              <lis />
            </cdr>
          </null-check>
        </cond-check>
        <cond-expr>
          <append>
            <qsort>
              <less/>
            </qsort>
            <cons>
              <car>
                <lis />
              </car>
              <qsort>
                <greater/>
              </qsort>
            </cons>
          </append>
        </cond-expr>
      </cond-clause>
      <cond-clause>
        <check>
          <true />
        </check>
      </cond-clause>
      <cond-expr>
        <let>
          <let-binding>
            <symbol name="pivot">
              <car>
                <lis/>
              </car>
            </symbol>
            <symbol name="x">
              <cadr>
                <lis/>
              </cadr>
            </symbol>
          </let-binding>
          <let-body>
            <if>
              <if-check>
                <greater-than-or-equal-to>
                  <x/>
                  <pivot/>
                </greater-than-or-equal-to>
              </if-check>
              <if-body>
                <then-clause>
                  <qsort>
                    <cons>
                      <pivot/>
                      <cddr>
                        <lis/>
                      </cddr>
                    </cons>
                    <cons>
                      <x/>
                      <less/>
                    </cons>
                    <greater/>
                  </qsort>
                </then-clause>
                <else-clause>
                  <qsort>
                    <cons>
                      <pivot/>
                      <cddr>
                        <lis/>
                      </cddr>
                    </cons>
                    <less/>
                    <cons>
                      <x/>
                      <greater/>
                    </cons>
                  </qsort>
                </else-clause>
              </if-body>
            </if>
          </let-body>
        </let>
      </cond-expr>
    </cond>
  </defun>
</xml>