why &drupal_static is getting used with refernce?
function drupal_static


  Provides central static variable storage.
  
  All functions requiring a static variable to persist or cache data
  within a single page request are encouraged to use this function
  unless it is absolutely certain that the static variable will not need
  to be reset during the page request. By centralizing static variable
  storage through this function, other functions can rely on a
  consistent API for resetting any other function's static variables.


More with examples: https://api.drupal.org/api/drupal/includes%21bootstrap.inc/function/drupal_static/7