func Fdiff(w io.Writer, a, b interface{}) {
	Pdiff(&wprintfer{w}, a, b)
}
Fdiff writes to w a description of the differences between a and b.
package pretty
import (
	"fmt"
	"io"
	"reflect"
)
type sbuf []string
func (p *sbuf) Printf(format string, a ...interface{}) {
	s := fmt.Sprintf(format, a...)
	*p = append(*p, s)
}
func Diff(a, b interface{}) (desc []string) {
	Pdiff((*sbuf)(&desc), a, b)
	return desc
}
type wprintfer struct{ w io.Writer }
func (p *wprintfer) Printf(format string, a ...interface{}) {
	fmt.Fprintf(p.w, format+"\n", a...)
}