anchor.diff:  
   The `do` operator calls a method on the client. For instance:
   
  +....
       - do:
           cluster.health:
               level: shards
  +....
   
   The response from the `do` operator should be stored in the `response` var, which
   is reset (1) at the beginning of a file or (2) on the next `do`.
  
positive.code: 
       - do:
         ... test definitions ...
  
  All tests in the file should be skipped if: `min <= current <= max`.
  
  
negative.code: If the arguments to `do` include `catch`, then we are expecting an error, which should
  be caught and tested.  For instance:
  
  ....
      - do:
          catch:        missing
          get:
              index:    test
              type:    test
              id:        1
  ....
  
  The argument to `catch` can be any of:
  
  [horizontal]
  `missing`::     a 404 response from ES
  `conflict`::     a 409 response from ES
  `request`::        a generic error response from ES
  