camlp5 parsing question

[Aaron Bohannon <bohannon@cis.upenn.edu>]

I don't understand why I am getting certain parse errors after
extending a grammar.  For instance, if I extend the original OCaml
grammar with these rules--

  expr: LEVEL "simple"
    [ [ "`"; "+"; "-" -> <:expr< 1 >> ]
    | [ "`"; "+"; "*" -> <:expr< 2 >> ] ];

--then when I try out the syntax, I get:

# ` + - ;;
- : int = 1
# ` + * ;;
Toplevel input:
# ` + * ;;
    ^^^
Parse error: '+' '-' expected after '`' (in [expr])

If the grammar is being left-factored according to the description in
the documentation (sec. 3.4 of
http://pauillac.inria.fr/~ddr/camlp5/doc/htmlc/grammars.html), I don't
see why these two rules should get in each other's way.  Moreover, the
base grammar defined in "pa_o.ml" seems to use rules with similarly
overlapping initial parts without any trouble.

 - Aaron