Modified the build rules for [calc-two] so as to avoid unused token warnings.

demos/calc-two/_tags

 <tokens.mlypack>: only_tokens
 <algebraic.mlypack>: external_tokens(Tokens)
 <reverse.mlypack>: external_tokens(Tokens)
+<reverse.mly>: unused_token(LPAREN)
+<reverse.mly>: unused_token(RPAREN)

demos/calc-two/myocamlbuild.ml

 open Ocamlbuild_plugin
 open Command
 
+
+(* Define ocamlbuild flags [only_tokens] and [external_tokens(Foo)] and
+   [unused_token(Bar)] which correspond to menhir's [--only-tokens] and
+   [--external-tokens Foo] and [--unused-token Bar]. When they are used, these
+   flags should be passed both to [menhir] and to [menhir --raw-depend]. *)
+
 let menhir_flags() =
-  (* Define two ocamlbuild flags [only_tokens] and [external_tokens(Foo)]
-     which correspond to menhir's [--only-tokens] and [--external-tokens Foo].
-     When they are used, these flags should be passed both to [menhir] and to
-     [menhir --raw-depend]. *)
   List.iter (fun mode ->
     
     flag [ mode; "only_tokens" ] (S[A "--only-tokens"]);
 
     pflag [ mode ] "external_tokens" (fun name ->
       S[A "--external-tokens"; A name]
+    );
+
+    pflag [ mode ] "unused_token" (fun name ->
+      S[A "--unused-token"; A name]
     )
       
   ) [ "menhir"; "menhir_ocamldep" ]