Camlp4 is a Pre-Processor-Pretty-Printer for Objective Caml. It offers tools
for syntax (grammars) and the ability to modify the concrete syntax of the
language (quotations, syntax extensions). 

The Objective Caml compiler has a special option "-pp", allowing to call 
any preprocessor. If Camlp4 is used, the parsing is done (only once) by
Camlp4, and the Objective Caml compiler resumes the rest of the compilation
(typing, code generation). 

Camlp4 can parse normal Ocaml concrete syntax or any other user-definable
syntax. As an example, an alternative syntax is provided, named revised,
because it tries to fix some small problems of the normal syntax.

Camlp4 can pretty print the normal Ocaml concrete syntax or the revised one.
It is therefore always possible to have a version of your sources
compilable by the compiler Objective Caml without preprocessing.