anchor.diff:  "function_score": {
       "query": {},
       "boost": "boost for the whole query",
  -    "FUNCTION": {},
  +    "FUNCTION": {}, <1>
       "boost_mode":"(multiply|replace|...)"
   }
   --------------------------------------------------
  +<1> See <<score-functions>> for a list of supported functions.
   
   Furthermore, several functions can be combined. In this case one can
   optionally choose to apply the function only if a document matches a
  
positive.code: 
  image::https://f.cloud.github.com/assets/4320215/768162/0b606884-e899-11e2-907b-aefc77eefef6.png[width="700px"]
  
  ===== Linear decay, keyword `linear`
  
  When choosing `linear` as the decay function in the above example, the
  
negative.code: 
  There are a number of options for the `field_value_factor` function:
  
  [horizontal]
  `field`::
  
      Field to be extracted from the document.
  
  `factor`::
  
      Optional factor to multiply the field value with, defaults to `1`.
  
  `modifier`::
  
      Modifier to apply to the field value, can be one of: `none`, `log`,
      `log1p`, `log2p`, `ln`, `ln1p`, `ln2p`, `square`, `sqrt`, or `reciprocal`.
      Defaults to `none`.
  
  `missing`::
  
      Value used if the document doesn't have that field. The modifier
      and factor are still applied to it as though it were read from the document.
  
  
   Keep in mind that taking the log() of 0, or the square root of a negative number
   is an illegal operation, and an exception will be thrown. Be sure to limit the
   values of the field with a range filter to avoid this, or use `log1p` and
   `ln1p`.
  
  [[function-decay]]
  ==== Decay functions
  
  Decay functions score a document with a function that decays depending
  