filters.go
func Filter(l LogBackend, r Ruler) LogBackend {
	return &filter{
		LogBackend: l,
		r:          r,
	}
}
func Filter(l LogBackend, r Ruler) LogBackend {
	return &filter{
		LogBackend: l,
		r:          r,
	}
}
func Filter ( l LogBackend , r Ruler ) LogBackend { return & filter { LogBackend : l , r : r , } }
// Filter creates a new filter with rules r for l backend.
Filter creates a new filter with rules r for l backend .