This macro expands by default to 
256. For a C++
      standard conforming preprocessor, the end-user can change this to
      512 or 1024 by defining this macro to either value before
      including any library headers, as long as the end-user also
      defines the 
BOOST_PP_LIMIT_MAG
      macro to be equal to or greater than this value. 
      
      NOTE: for the current latest versions of default preprocessor for
      Microsoft's VC++ compiler there is a nested macro limit of 256.
      This means in actuality that the number of repetitions using VC++
      is actually less than 256 depending on the repetition macro being
      used. For the BOOST_PP_REPEAT macro this limit appears to be 252
      while for the BOOST_PP_ENUM... series of macros this limit appears
      to be around 230, before the VC++ compiler gives a "C1009:
      compiler limit : macros nested too deeply" error. This is a
      compiler limitation of VC++ which may vary depending on the
      specific repetition macro being used, and therefore is not a
      problem the preprocessor library can solve. This limitation of the
      default preprocessor for VC++ does not exist for the new standard
      conforming preprocessor for VC++ in Visual Studio 2019 invoked
      with the
 /Zc:preprocessor switch.