Count members based on Analyzer filtered result

Sometimes you need to count members based on Analyzer filter selection, to do so we have to use a special set automatically generated by Analyzer internally. For example we have a table with 4 members before filtering:

After filter using Set Filter we now have only 3 members 

If we simply use .CurrentMember.Children it will still return us with 4 children under the parent

Count( [Product.Category].CurrentMember.Children )

But if we use the following expression we can then correctly identify the filtered members

Count( intersect([__Product.Category_FilterTree], [Product.Category].children))

Text in purple is the set automatically generated by Analyzer that has the information on filtered set, it always has the the format of [__Your Dimension.Attribute_FilterTree]