This predicate relates a given purposeful action of physically creating or destroying something to an unintended tangible output (see output) of that action. (by-product EVENT OBJECT) means that OBJECT is one of the outputs of EVENT, but not one of its intended outputs. For intended outputs, see product. Note that no output can be _both_ a product and a by-product of the same event, i.e. (mutually-negating predicate product by-product) holds.