The input variables of a half adder are called the augend and addend bits. Arithmetic was performed by a one-digit adder. The gate delay can easily be calculated by inspection of the full adder circuit. In the last chapter, he presented diagrams of several circuits, including a 4-bit full adder. This makes it practically useful as a parity generator or a modulo-2 adder. Each full adder requires three levels of logic. The simplest architecture is the ripple carry adder, which follows the standard multi-digit algorithm. One snake, the adder, is venomous but rarely deadly. However, using multiply-add-operations may prevent simultaneous add operation in one of the adder units. Each full adder inputs a "C", which is the "C" of the previous adder. The common adder, like most other vipers, is ovoviviparous. The membership consists of leader Cobra, Viper (Murtaugh), Eel, Death Adder (Scott), and Anaconda. Other signed number representations require more logic around the basic adder. Reptiles found include the European adder ("Vipera berus") and grass snake ("Natrix natrix"). The principal character is Edmund, Lord Blackadder, the great-grandson of the original Black Adder.