Being a template metaprogramming framework, the MPL concentrates on
getting one thing done well and leaves most of the clearly
preprocessor-related tasks to the corresponding specialized
libraries [PRE], [Ve03]. But whether we like it or not, macros play
an important role on today's C++ metaprogramming, and some of
the useful MPL-level functionality cannot be implemented
without leaking its preprocessor-dependent implementation
nature into the library's public interface.
