f (CallMethod "subtract" (Named xs))
        | Just (Number a) <- lookup "minuend" xs
        , Just (Number b) <- lookup "subtrahend" xs
        = return $ Number (a - b)
f (CallMethod "subtract" (Named xs))
        | Just (Number a) <- lookup "minuend" xs
        , Just (Number b) <- lookup "subtrahend" xs
        = return $ Number (a - b)
f (CallMethod "subtract" (Named xs))
        | Just (Number a) <- lookup "minuend" xs
        , Just (Number b) <- lookup "subtrahend" xs
        = return $ Number (a - b)