</code>, then the header value expressions may use attribute <code>x.output.f</code> to reference
the field value:</p><pre><code class=language-yaml>request_header_operations:
- name: x-istio-header
  values:
  - x.output.f
</code></pre><p>If the header value expression evaluates to an empty string, and the operation is to either replace
or append a header, then the operation is not applied. This permits conditional behavior on behalf of the
adapter to optionally modify the headers.</p><table class=message-