TDAI — Need to be able undo or unset recommendations

Tactical Decision Aids may have a revised opinion about previous decisions. This may mean lower confidence in a previous recommendation rather than an alternative recommendation (which would override). There needs to be a mechanism to unset, or recommend that the system reverts to the previous state.

Add Cancel Recommendation and Null Descriptor

Two additional mechanisms are proposed for different circumstances.
Firstly a Cancel Recommendation operation, which cancels the effect of the recommendation with the same request id. Not all recommendations will always be capable of being cancelled by the service implementor. For implementation compatibility purposes a 'can be cancelled' attribute is added to the response. As with referrals, additional responses are allowed for changes of state, so cancellation may become impossible later and, if so, the service provider sends an additional response to that effect.
Secondly a Null Descriptor should be defined, for categorization recommendations in particular, this recommends return to a state as if the Decision Aid has never made any recommendations for that entity