Indeed when some parameter are BOOST_PP_EMPTY the last parameter is
accessed correctly but results in a compilation error which doesn't stop
some compiler (e.g. g++) to go through the preprocessing pass and still
compile the binary correctly. But to avoid the error message I used a
workaround which behaves better. I preferred factoring it out to make
clear that there are some reason why this element is strangely accessed.
These macros are used to handle generation of boilerplate code or each
member to add to the adapted sequence. And in the commit
abff92ab65 we changed the signature of these
macros to handle generation of proxied object field type deduction, this
change simply fix the related macros.
