func (p *Prompter) Errf(format string, a ...interface{}) (n int, err error) {
	return fmt.Fprintf(p.Out, format, a...)
}
Errf is like fmt.Printf, except it writes to p.Err.
package interact
import (
	"fmt"
)
func (p *Prompter) Printf(format string, a ...interface{}) (n int, err error) {
	return fmt.Fprintf(p.Out, format, a...)
}
func (p *Prompter) Println(a ...interface{}) (n int, err error) {
	return fmt.Fprintln(p.Out, a...)
}