plone.app.discussion/plone/app/discussion/browser/captcha.zcml

<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:meta="http://namespaces.zope.org/meta"
    xmlns:zcml="http://namespaces.zope.org/zcml"
    i18n_domain="plone.app.discussion">

    <!--
      Plone 3 fixes

      Plone 3 / Zope 2.10 does not recognize the meta:provides feature.
      Therefore we claim to provide this feature when a suitable package is
      installed.
    -->
    <configure zcml:condition="installed plone.formwidget.captcha">
        <meta:provides feature="plone.app.discussion-captcha" />
    </configure>

    <configure zcml:condition="installed plone.formwidget.recaptcha">
        <meta:provides feature="plone.app.discussion-captcha" />
    </configure>

    <!-- Captcha comment form extender -->
    <configure zcml:condition="have plone.app.discussion-captcha">
        <!--
          Register the Captcha form extender and validator only if there are
          plugins installed that declare to implement a Captcha solution for
          plone.app.discussion (e.g. plone.formwidget.captcha and
          plone.formwidget.recaptcha).
        -->
        <adapter
            factory=".captcha.Captcha"
            provides="plone.app.discussion.interfaces.ICaptcha" />
        <adapter
            factory=".captcha.CaptchaExtender"
            provides="plone.z3cform.fieldsets.interfaces.IFormExtender" />
        <adapter
            factory=".validator.CaptchaValidator"
            provides="z3c.form.interfaces.IValidator"
            />
    </configure>

    <!-- Akismet Validator -->
    <configure zcml:condition="installed collective.akismet">
        <adapter
            factory="collective.akismet.validator.AkismetValidator"
            provides="z3c.form.interfaces.IValidator"
            />
    </configure>

</configure>