{ mqtt_servers.default = { host = "1.2.3.4", user = Some "username", pass = Some
"password" }
, layers.default
  =
  { default_mapping = Mapping.Print "no mapping!"
  , mappings =
      let sometopic = "homeassistant/switch/something/set"

      in  { key_0 =
              Mapping.Trigger (Action.MQTTPub { topic = sometopic, payload = "ON" })
          , key_1 =
              Mapping.Trigger
                (Action.MQTTPub { topic = sometopic, payload = "OFF" })
          }
  }
}