> This is a binary constraint. For this reason we are proposing that existence > is represented as an optional attribute with 2 values > 'required' and 'prohibited'.
Sam, an optional attribute with 2 values actually allows 3 states. Of course the default will map to one of the two values - but this does allow 2 ways of expressing the same concept. My two preferences for this situation (representing binary constraints) a) mandatory attribute with 2 values - or - b) an optional attribute with ONE possible value, and where the absence of the value is the default. Andrew
