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4. *I1:I2:I3If a signal of type I1 and a signal of type I2 are both present at time t, then the signal I3 is broadcast at the same time t unless I3 contains unquoted occurrences of the symbols  or singly quoted occurrences of *, in which case I3 is broadcast at time t + 1. |
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When the final string of any of these units (I2 for (1), (2), and (3), I3 for (4)) is interpreted for broadcast, one quote is stripped from each quoted symbol. |
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The concept of the state of a (finite) collection of broadcast units facilitates discussion of potential ambiguities in the actions and interactions of the four types of unit. This state at time t is, quite simply, the set of all signals present at time t, including the strings defining devices in the collection, the signals generated by those devices, and the signals generated in the environment of the collection (input signals). Thus the initial state is the set of strings used to specify the initial collection of units, together with all signals present initially. If we look again at the definition of type 4 broadcast units, we see that they may actually use signals in the current state to contribute additional signals to the current state (i.e., they can act with negligible delay much as the switching elements of computer theory). For example, given the broadcast units |
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with environmental (input) signals {100, 110} at t = 0 and {100} at t = 1, the state S(0) at t = 0 is |
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The latter signal in S(1), 11000, occurs because the unit  receives both the signal 110 and the signal 000 at t = 0, so that  and  , and hence the output 11000 occurs at time t = 1. A little thought shows that the instantaneous action of type 4 units does not interfere with the determination of a unique state at each time since type 4 units can add at most a finite number of signals to the current state. |
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Since the symbols  and  are meant only to designate initial or terminal strings their placement within arguments of a broadcast unit can be critical to unambiguous interpretation. For types 1 through 4, if I1 contains exactly one |
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