lter_actor</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">ChannelDescriptorT</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">SeverityDescriptorT</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">,</span> <span class="identifier">fallback_to_none</span><span class="special">,</spa