[Caml-list] Error in Camlp4 tutorial?

[Matt Gushee <mgushee@havenrock.com>]

Hello, all--

I am working through the Camlp4 Tutorial (which, by the way, I find a
very comprehensible treatment of a difficult subject -- thanks to M. de
Rauglaudre!), but I am having trouble with the add_infix function in
section 3.6.

I start the interactive toplevel with the command

  $ ocaml camlp4o.cma pa_extend.cmo

Then load the file "gr2.ml," which defines the 'expr' grammar entry (as
far as I can see, it's a faithful copy from section 3.4 of the
tutorial):

  # #use "gr2.ml";;

Then I attempt to write the add_infix definition, again, copying
verbatim from the tutorial, but I get the following error:

  # let add_infix lev op =
    EXTEND
      expr: LEVEL $lev$
        [ [ x = expr; $op$; y = expr -> <:expr< $lid:op$ $x$ $y$ >> ] ];
                                        ^^^^^^^
  Unbound quotation: "expr"

Can anyone help me understand what the problem is? Is there an error in
the tutorial, or have a done something wrong?

(P.S.: I don't really understand what '<:expr<' represents. Maybe the
answer would be obvious if I knew that?)
                                                          
-- 
Matt Gushee                 When a nation follows the Way,
Englewood, Colorado, USA    Horses bear manure through
mgushee@havenrock.com           its fields;
http://www.havenrock.com/   When a nation ignores the Way,
                            Horses bear soldiers through
                                its streets.
                                
                            --Lao Tzu (Peter Merel, trans.)

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