user_data for each of this's pads.
func will be called exactly once for each pad that exists at the time of this call, unless one of the calls to
false in which case we will stop iterating pads and return early. If new pads are added or pads are removed while pads are
being iterated, this will not be taken into account until next time this function is used.
a Element to iterate pads of
function to call for each pad
user data passed to