code: func Parent(sc model.SpanContext) SpanOption {
  	return func(t *Tracer, s *spanImpl) {
  		if sc.Err != nil {
  			// encountered an extraction error
  			switch t.extractFailurePolicy {
  			case ExtractFailurePolicyRestart:
  			case ExtractFailurePolicyError:
  				panic(s.SpanContext.Err)
  			case ExtractFailurePolicyTagAndRestart:
  				s.Tags["error.extract"] = sc.Err.Error()
  			default:
  				panic(ErrInvalidExtractFailurePolicy)
  			}
  			/* don't use provided SpanContext, but restart trace */
  			return
  		}
  		s.SpanContext = sc
  	}
  }
docs: // Parent will use provided SpanContext as parent to the span being created.