An example formal description of facets in ES

facets.txt

facets : "facets" : {
           <facet_def>
           [,<facet_def>]* 
         }

facet_def: <facet_name> : { 
             <facet_type>
             [,"global":true]
             [,<facet_filter>]
           }

facet_name: "[a-ZA-Z]+"

facet_type: <type_name> { 
              <type_body> 
            }

facet_filter: "facet_filter" : { 
                <filter_def> 
              }

// <> indicates required portions
// [] indicates optional things which is also a JS array, so it would be best to pick something else
// facet_type could instead link to drill-down and see all facet types and their formal description
// filter_def would link to the filters section