  In 1998, Zhang and Yeung found the first unconditional non-Shannon-type
information inequality. Recently, Dougherty, Freiling and Zeger gave six new
unconditional non-Shannon-type information inequalities. This work generalizes
their work and provides a method to systematically derive non-Shannon-type
information inequalities. An application of this method reveals new 4-variable
non-Shannon-type information inequalities.
