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Here is some additional detail describing how this would work:
For For the sake of this discussion lets we will call this new function AttributeInMetadata. This function would take the attribute on which it is defined and When this function is specified on a PermitValueRule element, it would check to see if he the enclosing attribute was listed in the requesting SPs AttributeConsumingService metadata element. . If there is a match, then the Rule returns TRUE. This type of match function could also have options specifying whether to release optional attributes and whether to filter on values too (since this is allowed in the metadata, but would be expensive to implement).
Given the above, currently fictitious, proposed matching function a site would do the following to use it.
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