When faced with a question of the sort, "Is schema X valid in class of frames C?", we usually go about proving or disproving this by assuming the antecedent of X and showing that the consequent follows from the relations that characterize C. Simply enough.
However, I am faced with proving that the schema □(□A -> A) is valid in the class of all secondary reflexive frames and am not sure about how to proceed, as there is no simple antecedent to assume or consequent to aim for. I have thought about using K to decompose the above but that would allow for only a partial proof.
Note: I am not looking for a proof but rather for an explanation of methodology.