func (pcs Trace) Format(s fmt.State, c rune) {
	s.Write([]byte("["))
	for i, pc := range pcs {
		if i > 0 {
			s.Write([]byte(" "))
		}
		pc.Format(s, c)
	}
	s.Write([]byte("]"))
}
func (pcs Trace) Format(s fmt.State, c rune) {
	s.Write([]byte("["))
	for i, pc := range pcs {
		if i > 0 {
			s.Write([]byte(" "))
		}
		pc.Format(s, c)
	}
	s.Write([]byte("]"))
}
// Format implements fmt.Formatter by printing the Trace as square brackes ([,
// ]) surrounding a space separated list of Calls each formatted with the
// supplied verb and options.