-- | Modify a PVar, strictly.
modifyPVar' :: PVar a -> (a -> a) -> VTx ()
modifyPVar' var f = do
    x <- readPVar var
    writePVar var $! f x

modifyPVar' :: PVar a -> (a -> a) -> VTx ()
modifyPVar' var f = do
    x <- readPVar var
    writePVar var $! f x

modifyPVar' var f = do
    x <- readPVar var
    writePVar var $! f x
