def trace
      audit_id = self.respond_to?(:payload) && payload.is_a?(Hash) && (payload['audit_id'] || payload[:audit_id])
      tok = self.respond_to?(:token) && token
      tr = "<#{audit_id || nil}> <#{tok}>"
    end
def trace
      audit_id = self.respond_to?(:payload) && payload.is_a?(Hash) && (payload['audit_id'] || payload[:audit_id])
      tok = self.respond_to?(:token) && token
      tr = "<#{audit_id || nil}> <#{tok}>"
    end
Generate token used to trace execution of operation across multiple packets

 === Return
 tr(String):: Trace token, may be empty