Comment on the `uPred` module trick.

theories/base_logic/base_logic.v

 From iris.base_logic Require Export derived.
 Set Default Proof Using "Type".
 
+(* The trick of having multiple [uPred] modules, which are all exported in
+another [uPred] module is by Jason Gross and described in:
+https://sympa.inria.fr/sympa/arc/coq-club/2016-12/msg00069.html *)
 Module Import uPred.
   Export upred.uPred.
   Export primitive.uPred.