gistfile1.txt

class M {
    protected foo() { println 'foo' }
}

@Mixin(M) class A {
 def bar() { foo() }
}

class B extends A {}

def b = new B()
def a = new A()
a.bar() //<-- comment out this line and see the difference
b.metaClass.foo = {println 'winning'}
b.bar()