Zero element

The zero element in relational network notation is represented by a small circle at the end of a line. Impulses moving to a zero element disappear; and an impulse may move from a zero element to the connecting line at any time.

Example
In the diagram, four examples of the zero element occur.
 * The leftmost zero element, below a downward OR node, indicates that instead of "always" or "often", there may be a null output, i.e., nothing.
 * The next zero element shows that there could be "have-en" or nothing. In other words, the 'have-en' is optional.
 * The next zero element, above the downward AND node, shows the (optional) third of three possible outputs: (1) be-ing-go-to followed by have-en; (2) be-ing-go-to followed by nothing; (3) nothing at all.
 * The rightmost zero element shows three possibilities: be-ing, be-able-to, or nothing.