I'm trying to learn how datascript/datomic rules work.

(def conn (ds/create-conn {}))
  
  (ds/transact! conn
    [{:id 1 :size 2}
     {:id 2 :size 3}])

  (def query '[:find ?size
               :in $ % ?id
               :where
               (size ?id ?size)])
  
  (ds/q query (ds/db conn)
    '[[(size ?id ?size)
       [?x :id ?id]
       [?x :size ?size]]]
    1)
  ;;returns #{[2]} (expected)

  (ds/q query (ds/db conn)
    '[[(size [?id] ?size) ;; require id be bound by enclosing in vector
       [?x :id ?id]
       [?x :size ?size]]]
    1)
  ;; returns #{[3] [2]}. not sure why.