anchor.diff:  (def em2 (entity-map! [:A/B :C/D Foo] m)) ;; long keychains ok too
   ----
   
  -### field types
  +===# field types
   
   The value part of an entity-map is just a map.  The datastore
   restricts the permissible value types; see  link:https://cloud.google.com/appengine/docs/java/datastore/entities#Java_Properties_and_value_types[Properties and value types].
  
positive.code: push constructor, like so: `(entity-map! [:A] {:a 1})`; to fetch
  Entities by kind, do the same with the pull constructor: `(entity-map*
  [:A])`.  Kinded construction is not supported for the local
  constructor (`entity-map`); the datastore can only generated Ids for
  stored entities.
  
  [source,clojure]
  
negative.code:  (entity-map* :pfx [:A/B :C/D]) ;; fetch all entities with keychain prefix (i.e. ancestor) [:A/B :C/D]
  ----
  
  === mutation
  
  [[app-listing]]
  [source,clojure]
  