code: def child(self, name):
          
          trace = self.__class__(
              name, trace_id=self.trace_id, parent_span_id=self.span_id)
          trace.set_endpoint(self._endpoint)
  
          return trace
docs: Create a new instance of this class derived from the current instance
          such that:
  
              (new.trace_id == current.trace_id and
               new.parent_span_id == current.span_id)
  
          The new L{Trace} instance will have a new unique span_id and if set the
          endpoint of the current L{Trace} object.
  
          @param name: C{str} name describing the new span represented by the new
              Trace object.
  
          @returns: L{Trace}