code: func (cs CallStack) Format(s fmt.State, verb rune) {
  	s.Write(openBracketBytes)
  	for i, pc := range cs {
  		if i > 0 {
  			s.Write(spaceBytes)
  		}
  		pc.Format(s, verb)
  	}
  	s.Write(closeBracketBytes)
  }
docs: // Format implements fmt.Formatter by printing the CallStack as square brackets
  // ([, ]) surrounding a space separated list of Calls each formatted with the
  // supplied verb and options.