This predefined Solver constraint forces a constrained set variable not to contain a given element. It does so by creating a constraint that acts on the constrained set variable var. When you post IlcMember, it prevents the value of var from containing element. The following cases can arise:

• If element belongs to the required set of var, then failure occurs.
• If element does not belong to the possible set of var, then nothing happens.
• In any other case, element is removed from the possible set of var, and the constraints posted on var are activated. If the number of required elements becomes equal to the number of possible elements as a result of this operation, the value of var becomes the required set.

This predefined Solver constraint forces a constrained integer expression not to be an element of the array of integers indicated by elements. When you post IlcNotMember, it does the opposite of IlcMember.