counter

type Counter interface {
	// Add increments the value for key by delta.
	Add(key interface{}, delta int)

	// Values returns the counted values.
	Values() M

	// Diff returns the difference between the counted values and m. A return
	// value of nil indicates no difference.
	Diff(m M) []Difference
}

type Difference struct {
	Key  interface{}
	Got  int
	Want int
}

type M map[interface{}]int