anchor.diff:  `in`.  We can easily support these using metadata syntax.  For `in`,
   use `some`.
   
  -==== key filters
  +=== [[keys]] key filters
   
   E.g. fetch entities with kind :Person whose key is greater than some k K:
   
  
positive.code: (entity-map* :parent [:A/B :Person])  ;; parent query: all :Person ems with parent [:A/B]
  ----
  
  ==== property filters
  
  ==== inequality filters
  
  Metadata syntax like `^>{:a 3}` (meaning all :a vals > 3) applies to
  the entire map.  But since the datastore restricts inequality filters
  
negative.code:     ;;       ems2 (ds/emaps?? [:A] (| :a = 1 ; or
      ;;                                :b = 2))]
  
  == [[mutation]] mutation
  
  * support mode tags :into, :merge, etc.
  
  