able design
             would use periodic publishers, <em>i.e.</em>
             publishers that send data at a constant rate,
             even if no new data are available.
             The data update, in this case,
             would have the effect of a heartbeat.
</li></ul><p>The library uses a set of headers that must not be used by applications.
  All internal header keys start and end with two underscores.
  By avoiding this naming of header keys, application code easily avoids
  naming conflicts