BOOST_VMD_IS_VMD_SEQ — Determines if a sequence is a VMD seq.
The macro checks that the sequence is a VMD seq. A VMD seq, which may be a Boost PP seq or emptiness, is a superset of a Boost PP seq. It returns 1 if it is a VMD seq, else if returns 0.
sequence = a possible Boost PP seq
returns = 1 if it a VMD seq, else returns 0.