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''housekeeping" units such as  , which puts the sum in the same form as the addends, are required to start up the process, keep track of position, etc. The overall process is simply a straightforward extension of techniques already illustrated. |
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7. As a final example note that any string identified with a suffix I can be reproduced by the broadcast unit  .Note additionally that this unit itself has the suffix I! Hence, if we start with this unit alone, there will be 2t copies of it after t time-steps. By revising the unit a bit, so that its action is conditional on a signal J,  ,this self-reproduction can be controlled from outside (say by other broadcast units). By extending this idea, with the help of the techniques outlined previously, we can put together a set of broadcast units which reproduces an arbitrary set of broadcast units (including itself). The result is a self-reproducing entity which can be given any of the powers expressible in the "broadcast language." |
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At this point it would not be difficult to give the "broadcast language" a precise, axiomatic formulation, developing the foregoing examples into a formal proof of its powers. (For anyone familiar with the material presented, say, in Arbib [1964] or Minsky [1967] this turns out to be little more than a somewhat tedious exercise.) However, our present objectives would be little advanced thereby. It is already reasonably clear that the "broadcast language" exhibits the desiderata outlined at the beginning of section 2. In particular, the broadcast units satisfy the functional integrity requirement (2) in a straightforward way. Consequently, strings of broadcast units can be manipulated by generalized genetic operators with attendant advantages vis-à-vis schemata (see section 6.3 and the close of chapter 7). Moreover a little thought shows that by using the techniques of usage (4) along with those of (2), units can be combined to define a crossover operator which acts only at specified "punctuations" (such as * s or : s or at a particular "indicator" string I). The other generalized genetic operators can be similarly defined. New detectors can be formed naturally from environmental signals (represented as binary strings). For example, a signal can be converted to an argument which will detect similar signals (elements of a superset) simply by inserting "don't cares" ( ) at one or more points. Thus,  converts any signal with prefix E ("environmental") into a permanent piece of data which can then be manipulated as in usages (4) and (6) to form a new broadcast unit with some modification of the signal as an argument. |
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The collection of broadcast units employed by an adaptive system at any time will, in effect, determine its representation of the environment. Since the units themselves are strings which can be manipulated by generalized genetic operators, strings of units ("devices") can be made subject to reproductive plans and intrinsic |
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