                                       source diversity, for free
            . . Lexical route . . . . . . . . . . . . . . . . . . .
            :  +-----------+    +---------------+   +===========+  :
       +------>| BM25      |--->| forward index |--># one hit   #  :
       |    :  | k1=1.5,   |    | DF at query   |   # per       #  :
       |    :  | b=0.75    |    | time          |   # document  #  :
       |    :  +-----------+    +---------------+   +===========+  :
  +-------+ . . . . . . . . . . . . . . . . . . . . . . . . . . . .
  | query |
  +-------+ . . Dense route . . . . . . . . . . . . . . . . . . . .
       |    :  +------------+   +--------------+    +-----------+  :
       +------>| bi-encoder |-->| vector store |--->| pool      |  :
            :  | 200 / 40   |   | 40,741       |    | k x 4 = 20|  :
            :  | chunks     |   | chunks       |    |           |  :
            :  +------------+   +--------------+    +-----------+  :
            :       +------------------------------------+         :
            :       v                                              :
            :  +- - - - - - - -+       +---------+  +-----------+  :
            :  | cross-encoder |..off.>| cap     |->| top k     |  :
            :  | rerank -- off |       | 3 per   |  | k = 5     |  :
            :  +- - - - - - - -+       | source  |  +-----------+  :
            :                          +---------+                 :
            . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 ceiling: for 27% of queries the correct paper never enters the pool
