Require Attribute rule not getting overidden by exclude rule

Hi All,

I have two rules:

1- When a=Y requires selection of any item from class A

2- When a=N excludes selection of an item from class A

When i try to change the value of attribute a from Y to N,the configurator does not allow me to do that and give s a conflict. which kind of rule should I create that excludes should override the require rule and the products of first rule belonging to class A if added could be deleted when a=N.

Thanks

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Sanket