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The addition of nonextensible constraints prevents redundant constraints by means of a corresponding NAC in P_{nonextensiblea}-productions. This avoidance of redundancy is not ensured for extensible constraints. Generic structural constraints can overcome this problem.

A production for extending an extensible constraint P_extensiblee adds further edges to the graph. Extensible constraints of the same type must differ by involving at least one other (or additional) element. There must be a structural constraint S_extensible for each extensible constraint type, docking at each pair of potentially redundant constraints, and having all possible distinctive features as test alternatives.

Potentially redundant constraints are two constraints of the same type that are connected to the same elements according to the base graph p_base:

Construction S_extensible - docking part: The docking part of the structural constraint is created by two p_base graphs, where the corresponding vertices (without the constraints themselves) are unified. To avoid multiple redundant structural constraint instances per potentially redundant constraint pair, all but the constraints themselves are a PAC.

Possible distinctive features are vertices that are connected to one constraint but not (in the same way) to the other:

Construction S_extensible - testing part: There are two alternatives per unique edge (label; direction with respect to the constraint - toward it or away from it) that is included in the constraint's extension graphs. Each of the two alternatives consists of a graph with the two constraints of the docking part and an additional general vertex. Between each constraint and the general vertex is an edge corresponding to the unique edge. In the one alternative, the edge to the first constraint is an NAC; in the other alternative, the edge to the second constraint is an NAC.

The figure below shows an example of a Subset constraint that forces the set of ()-connected variables to be a subset of the set of ()-connected variables. The p_base graph of a Subset constraint includes two variables (), and there are two possible extensions corresponding to the two possible variable connections.

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Last update:
May 20, 2001 by Alexander Nareyek