event-driven simulation

In section 3.3.4, SICP presents a digital circuit simulator that implements it using a design called event-driven programming. In such designs basically actions("events") triggers further events that happen at a later time and those in turn trigger further events and so on.
To be particular about the example given in the book, Wire is the main object where we can set signal and can add any number of events(no-arg procedures). Whenever a signal changes(event of signal change happens), it triggers all the events added to it and those events when happen change signal on other wires which in turn trigger more events and so on... that is how the signal propagates.

I also wrote a port of this example in scala here.