diff --git a/_tags b/_tags
index 2e45667..8a23c20 100644
--- a/_tags
+++ b/_tags
@@ -1,4 +1,4 @@
-<global> or <utilities> or <minils> or <heptagon>:include
+<global> or <utilities> or <minils> or <heptagon> or <main>:include
 <**/*.ml>: debug, dtypes
 <preproc.ml>: camlp4of, use_camlp4
 <**/*.{byte,native}>: use_unix, use_str, debug
diff --git a/myocamlbuild.ml b/myocamlbuild.ml
index 4806e8b..cc4da2a 100644
--- a/myocamlbuild.ml
+++ b/myocamlbuild.ml
@@ -1,12 +1,7 @@
 open Ocamlbuild_plugin
 open Ocamlbuild_plugin.Options
 
-let sub_dirs = ["global"; "parsing"; "sigali"; "dataflow"; "sequential";
-                "analysis"; "translation"; "main"; "simulation"]
-
 let df = function
-  | Before_options ->
-      include_dirs := sub_dirs @ !include_dirs
   | After_rules ->
       (* Tell ocamlbuild about the camlp4 library. *)
       ocaml_lib ~extern:true ~dir:"+camlp4" "camlp4";
diff --git a/preproc.ml b/preproc.ml
new file mode 100644
index 0000000..6529af7
--- /dev/null
+++ b/preproc.ml
@@ -0,0 +1,56 @@
+(** {1 Micro pre-processor for Heptagon}
+
+    This module uses camlp4 to replace some fixed strings by string literals at
+    compile-time. At the moment, we only replace DATE by the current date and
+    STDLIB by "../../lib". Each pseudo-variable can be overriden by the
+    environment variable of the same name. *)
+
+open Camlp4.PreCast
+open Unix
+
+(** {2 Compile-time strings} *)
+
+(** [date] is a string denoting the current date. *)
+let date =
+  let days = [| "sunday"; "monday"; "tuesday"; "wednesday"; "thursday"; "friday";
+                "saturday" |]
+  and months = [| "january"; "february"; "march"; "april"; "may"; "june";
+                  "july"; "august"; "september"; "october"; "november";
+                  "december" |] in
+
+  let tm = Unix.localtime (Unix.gettimeofday ()) in
+
+  let (day, month) =
+    let prefix s = String.sub s 0 3 in
+    (prefix days.(tm.tm_wday), prefix months.(tm.tm_mon)) in
+
+  Printf.sprintf "%s. %s. %d %d:%d:%d CET %d"
+    day month tm.tm_mday tm.tm_hour tm.tm_min tm.tm_sec (1900 + tm.tm_year)
+
+
+(** [stdlib] is the location of the standard Heptagon library. *)
+let stdlib =
+  let wd = Unix.getcwd () in
+  Filename.concat (Filename.dirname (Filename.dirname wd)) "lib"
+
+(** Association list defining bindings between constant and our "compile-time
+    constants". *)
+let env = [("DATE", date); ("STDLIB", stdlib)]
+
+(** {2 Camlp4 hook} *)
+
+(** Our home-grown super-duper syntax filter. Looks for string constants present
+    in [subst] and replaces them according to the couple found in the
+    environment defined above. *)
+let filter =
+  object
+    inherit Ast.map as super
+    method expr e = match e with
+      | <:expr< $str:s$ >> when List.mem_assoc s env ->
+        let repl = try Sys.getenv s with Not_found -> List.assoc s env in
+        <:expr@here< $str:repl$ >>
+      | x -> x
+  end;;
+
+(** Tell Camlp4 about it. *)
+AstFilters.register_str_item_filter filter#str_item