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This is the tracker item for the proposal for an SBML Level 3 package to support the indication of required elements.
A draft specification for the 'req' package is now available at http://sbml.svn.sourceforge.net/viewvc/sbml/trunk/specifications/sbml-level-3/version-1/req/specification/
Packages as a whole ended up being dealt with in a completely different manner than being tracked here.