“The methods I’ve described can be used to implement any function that stays constant in time, but a more interesting class of functions are those that involve sequences in time. To handle such functions, we use a device called a finite-state machine. Finite-state machines can be used to implement time-varying functions—functions that depend not just on the current input but also on the previous history of inputs. Once you learn to recognize a finite-state machine, you’ll notice them everywhere—in combination locks, ballpoint pens, even legal contracts. The basic idea of a finite-state machine is to combine a look-up table, constructed using Boolean logic, with a memory device. The memory is used to store a summary of the past, which is the state of the finite-state machine.”