anchor.diff:  
   ==== Missing Values
   
  -Numeric fields support specific handling for missing fields in a doc.
  -The `missing` value can be `_last`, `_first`, or a custom value (that
  +The `missing` parameter specifies how docs which are missing
  +the field should be treated: The `missing` value can be
  +set to `_last`, `_first`, or a custom value (that
   will be used for missing docs as the sort value). For example:
   
   [source,js]
  
positive.code: --------------------------------------------------
  {
      "sort" : [
          { "post_date" : {"order" : "asc"} },
          "user",
          { "name" : "desc" },
          { "age" : "desc" },
  
negative.code: 
  Elasticsearch supports sorting by array or multi-valued fields. The `mode` option
  controls what array value is picked for sorting the document it belongs
  to. The `mode` option can have the following values:
  
  [horizontal]
  `min`:: Pick the lowest value.
  `max`:: Pick the highest value.
  `sum`:: Use the sum of all values as sort value. Only applicable for
          number based array fields.
  `avg`:: Use the average of all values as sort value. Only applicable
          for number based array fields.
  
  