The BOOST_PP_TUPLE_ELEM macro extracts an element from a tuple.
BOOST_PP_TUPLE_ELEM(size, i, tuple)
The size of the tuple.
Valid tuple sizes range from 0 to BOOST_PP_LIMIT_TUPLE.
The zero-based index into the tuple of the element to be extracted.
Valid values range from 0 to size - 1.
The tuple from which an element is to be extracted.
The size argument must be the actual size of the tuple,
and i must be less than the size of the tuple.