Behaviour of <attribute> and <attributeGroup> in <restriction> element

Can anyone explain to me how <attribute> and <attributeGroup> work when they appear inside a <restriction> in a type defintion please?

Are they used to restrict the permissible content of an attribute, the attributes in an attribute group? In particular can it be used to prohibit a specific attribute that was formerly permitted before the restriction was imposed (actually I hope this is NOT a possible use - the examples given in the primer seem to indicate I am right by virtue of the fact that they don't address this possibility)? Can someone provide a few examples of the correct syntax and a description of the sematics or point me to some relevant literature? I'm afraid I find the wording of the specification a bit hard to work through on this subject

Many thanks

Hugh

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