I started from the wiki page describing how to use AST filters:
http://brion.inria.fr/gallium/index.php/Camlp4MapGenerator

The example doesn't work in the toplevel (3.10.1), although it works 
otherwise. How to make it work?

Here is my testing session:

$ cat simplify.ml
open Camlp4.PreCast

let simplify = Ast.map_expr begin function
   | <:expr< $x$ + zero >> | <:expr< zero + $x$ >> -> x
   | x -> x
end
in AstFilters.register_str_item_filter simplify#str_item

let _ =
   prerr_string "Loaded \"simplify\"\n";
   flush stderr

$ ocamlc -c -pp camlp4of -I +camlp4 simplify.ml

$ \ocaml -I +camlp4 camlp4lib.cma simplify.cmo
Loaded "simplify"
         Objective Caml version 3.10.1

# 1 + zero;;
Unbound value zero
#

$ camlp4o ./simplify.cmo -str '1 + zero'
Loaded "simplify"
1




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