As I wrote to the list earlier, I have noticed that the 
Camlp4MacroParser in 3.10.0+beta has an unfortunate problem - when 
processing an "IFDEF" (or "IFNDEF") directive, it will execute the 
"DEFINE"s in _both_ branches, regardless of whether the test is true or 
not (this was caused by the fact that the DEFINE directives would be 
executed eagerly at parse-time).

Attached is a corrected and improved version of the Camlp4MacroParser:

- Fixes the above problem; now the execution of the "DEFINE" directives 
inside IFDEF/IFNDEF is correct.

- Adds support for "local" definitions
   DEFINE <lident> = <expression> IN <expression>
   This is particularly useful for things like

   DEFINE body =
     ...
   IN
   IFDEF DEBUG
     let result = body in
     print_debug result;
     result
   ELSE
     body

- Adds an ability to omit the "ELSE" part of the IFDEF/IFNDEF 
expressions (the "ELSE" branch is then taken to default to unit 
expression "()").

- Adds an ability to use the macro argument as a pattern (provided the 
argument is a sufficiently simple expression)

- Adds a special macro "NOTHING" that can be used to "wipe out" unneeded 
function arguments. This is useful to be able to generate functions of 
different arity using the same macro.

The improved Camlp4MacroParser.ml is attached.

Aleksey