copy/copy.go
func (c *copier) Printf(format string, a ...interface{}) {
	fmt.Fprintf(c.reportWriter, format, a...)
}
func (c *copier) Printf(format string, a ...interface{}) {
	fmt.Fprintf(c.reportWriter, format, a...)
}
// Printf writes a formatted string to c.reportWriter.
// Note that the method name Printf is not entirely arbitrary: (go tool vet)
// has a built-in list of functions/methods (whatever object they are for)
// which have their format strings checked; for other names we would have
// to pass a parameter to every (go tool vet) invocation.