title: Asymmetric Decay Function
body: <p>I have an elasticsearch Index containing some Documents, where each Document may have a field called "creationDate" which contains a date.</p>
  
  <p>Now I'm querying the Index with a date, and I want to score Documents according to creationDate so I set up a gaussian decay function.</p>
  
  <p>However as far as I understand it the decay functions are all symmetrical: i.e. if i have a document from 2005 one from 2007 and search for 2006 both documents will get the same score.
  However I'd like to be able to use different decays for Values less than the searchterm than for those that are greater.</p>
  
  <p>Is there a built in way to achieve this?</p>