[boost] [mpl] RFE: accessing ith tail of [reverse_]fold

I just uploaded to boost vault: inherit_tail_type.zip which contains inherit_tail_type.hpp and a test. It allows easy access to ith tail in fold<types, empty_base, inherit2<_2,_1> >. It has specializations for the tail being on the left or right side of inherit2, and for using reverse_fold also. I'm proposing this as an enhancement to mpl.