anchor.diff:  Availability = Alive Rate
   
   [[freshness]]
  -Freshness
  -~~~~~~~~~
  +=== 2.2 Freshness
   
   Freshness represents how often data in the endpoint is updated. We
   measure the following metrics for freshness:
  
positive.code:   \right.$]
  
  [[validity]]
  Validity
  ~~~~~~~~
  
  Validity represents how endpoint and data in it obey the rules. We
  measure the two metrics:
  
negative.code: In Section [metrics], we show how to score endpoints from each aspect.
  
  [[metrics]]
  == 2. Metrics of Umaka Score
  
  [[availability]]
  === 2.1 Availability
  
  Availability represents the degree of ready for use. High availability
  value means we can access the endpoint most of all the time. Low
  