15. The system of claim 10 , wherein receiving one or more rules comprises receiving a pattern definition including the one or more rules and wherein evaluating the one or more rules comprises evaluating the pattern definition in conjunction with one or more additional pattern definitions, and wherein each of the one or more additional pattern definitions comprises one or more rules comprising one or more declarative statements specifying one or more conditional diagram layout patterns based on one or more attributes of diagram elements.
15. The system of claim 10 , wherein receiving one or more rules comprises receiving a pattern definition including the one or more rules and wherein evaluating the one or more rules comprises evaluating the pattern definition in conjunction with one or more additional pattern definitions, and wherein each of the one or more additional pattern definitions comprises one or more rules comprising one or more declarative statements specifying one or more conditional diagram layout patterns based on one or more attributes of diagram elements. 17. The system of claim 15 , wherein evaluating the one or more rules using one or more attributes of one or more elements of a diagram is performed as a result of selectively triggering the pattern definition.