Currently Boost.Predef will define multiple symbols when, for example, a compiler "emulates" another compiler. The current common occurrence of this is the GCC compiler which many other compilers emulate by defining the GCC version macros. This is no longer the case for the OS definitions though. The OS definitions are mutually exclusive. And hence in order to make for a more consistent behavior I am considering changing the library to make not just OS defs mutually exclusive. But also making compiler, library, and architecture mutually exclusive. And I'm asking for some general feedback as to what the most useful approach, from using Boost.Predef, is likely to be. In particular I'm considering the following choices:

1. Simple mutual exclusion of BOOST_ARCH_*, BOOST_COMP_*, BOOST_LIB_C_*, BOOST_LIB_STD_*, and of course BOOST_OS_* definitions.

2. Mutual exclusion of the base/real definitions with the addition of *_EMULATED definitions when multiple defs detected. For example for GCC we would have BOOST_COMP_GCC_EMULATED indicated the version of the GCC being emulated.

3. No mutual exclusion of base definitions but with the addition of a "flag" definition when multiple defs are detected. For example with GCC it would have the usual BOOST_COMP_GCC but also have a "#define BOOST_COMP_GCC_IS_EMULATED".

4. Do nothing and live with multiple definitions and the current discrepancy in behavior for BOOST_OS definitions.

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