fmodify :: Field rec fld -> (fld -> fld) -> rec -> rec
fmodify field f x = fset field (f $ fget field x) x
fmodify :: Field rec fld -> (fld -> fld) -> rec -> rec
fmodify field f x = fset field (f $ fget field x) x
fmodify field f x = fset field (f $ fget field x) x