BOOST_VMD_IS_PARENS_EMPTY — Determines if the sequence is a set of parens with no data.
sequence = a VMD sequence
returns = 1 if the sequence is a set of parens with no data, else returns 0.
A set of parens with no data may be:
1) a tuple whose size is a single element which is empty or
2) a single element seq whose data is empty