[1] A full-adder is a basic circuit element used in adding two binary numbers. Here A and B are the bits at corresponding positions in the two numbers to be added, and C$_{in}$ is the carry bit from the addition one place to the right. The circuit generates SUM, which is the sum bit in the corresponding position, and C$_{out}$, which is the carry bit to be propagated to the left.
[2] These functions are simply syntactic sugar that allow us to use ordinary functional syntax to access the local functions of objects. It is striking that we can interchange the role of functions and data in such a simple way. For example, if we write wire('get_signal') we think of wire as a function that is called with the message "get_signal" as input. Alternatively, writing get_signal(wire) encourages us to think of wire as a data object that is the input to a function get_signal. The truth of the matter is that, in a language in which we can deal with functions as objects, there is no fundamental difference between functions and data, and we can choose our syntactic sugar to allow us to program in whatever style we choose.
[3] The agenda is a headed list, like the tables in section 3.3.3, but since the list is headed by the time, we do not need an additional dummy header (such as the *table* symbol used with tables).
[4] Observe that the if expression in this function has no alternative expression. Such a one-armed if statement is used to decide whether to do something, rather than to select between two expressions. An if expression returns an unspecified value if the predicate is false and there is no alternative.
[5] In this way, the current time will always be the time of the action most recently processed. Storing this time at the head of the agenda ensures that it will still be available even if the associated time segment has been deleted.
3.3.4 A Simulator for Digital Circuits