code: def make_single_header(context: TraceContext) -> Headers:
      
      # b3={TraceId}-{SpanId}-{SamplingState}-{ParentSpanId}
      c = context
  
      # encode sampled flag
      if c.debug:
          sampled = 'd'
      elif c.sampled:
          sampled = '1'
      else:
          sampled = '0'
  
      params = [c.trace_id, c.span_id, sampled]  # type: List[str]
      if c.parent_id is not None:
          params.append(c.parent_id)
  
      h = DELIMITER.join(params)
      headers = {SINGLE_HEADER: h}
      return headers
docs: Creates dict with zipkin single header format.