[ A -> B |- ~ B -> ~ A ]
------------------------------ Implication strategy
[ A -> B , ~ B |- ~ A ]
------------------------------ Negation rewrite
[ A -> B , ~ B |- A -> _|_ ]
------------------------------ Implication strategy
[ A , A -> B , ~ B |- _|_ ]
------------------------------ Modus ponens
[ A , A -> B , B , ~ B |- _|_ ]
------------------------------ Forgetting
[ B , ~ B |- _|_ ]
------------------------------ Ex falso quodlibet _|_
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