http://bluestorm.info/camlp4/pa_where/list.php

Use :
     Introduce a "where" keyword for backward declarations.

     Generic form :
       <something> where <declaration>  ==>  <declaration> (sep) <something>
     Supported forms : where let, where val
     Examples :
       expr where let a = b     ==>  let a = b in expr
       expr where let rec a = b ==>  let rec a = b in expr
       str_item where val a = b ==>  let a = b ;; str_item 
   
     Default case : as "where let" is the more common form,
     the "let" is optional and you can use "where" alone :
       expr where a = b  ==> let a = b in expr