All objects have both identity (the object's location in memory) and state (the object's data).
This generic class allows the developer to easily implement identity and state comparison
for each object
See
http://journal.r-project.org/archive/2011-1/RJournal_2011-1_Kane~et~al.pdf
see http://dl.dropbox.com/u/3001534/engine.c
see http://www.mathworks.com/matlabcentral/fileexchange/28156-limit-order-book-simulation