in lib/nano.js [600:612]
  function enableReplication (source, target, opts0, callback0) {
    const { opts, callback } = getCallback(opts0, callback0)
    if (missing(source, target)) {
      return callbackOrRejectError(callback)
    }
    // _replicator
    opts.source = _serializeAsUrl(source)
    opts.target = _serializeAsUrl(target)
    return relax({ db: '_replicator', body: opts, method: 'POST' }, callback)
  }