XAML Expression Blend Button Conditional Statement or Other trigger?

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expression

Blend

I am attempting to recreate the minimize, maximize/restore and close button.
I have successfully created the minimize and close button; however I want the style to change for the maximize restore button IF the application is maximised.

I was thinking something along the lines of a conditional statement
i.e.
IF WindowState is Maximized Buttonx Style equals 0; ELSE equals 1?

Well something on the basis of that, any help is welcome.