%% slac-pub-7205: page file slac-pub-7205-0-0-2-3.tcx.
%% subsection 2.3 Program Universe [slac-pub-7205-0-0-2-3 in slac-pub-7205-0-0-2: L$,
the portion of all string of length $L$ changes
only by discrimination between members of this collection.
Consequently it can end up containing at most $2^{n_L}-1$ types of distinct,
non-null strings no matter how much longer program universe runs.
Whether it ever even reaches this bound, and the value of $n_L$ itself,
are {\it historically contingent} on which run of program universe
is considered. This observation provides a model for {\it context sensitivity}.
One result of this feature of program universe is that
at any later stage in the evolution of the
universe we can always separate any string into two portions,
a {\it label string} ${\bf N}_i(L)$ and a {\it content string}
${\bf C}_i(S-L)$ and write
$P_i(S) ={\bf N}_i(L)\Vert {\bf C}_i(S-L)$ with $i \in 1,2,....,n_L$,
making the context sensitivity explicit..
Once we separate labels from content, the permutation invariance we talked about
above can only be applied to the columns in the label and/or to the
columns in the content parts of the strings separately.
Permutations which cross this divide will interfere with any
physical interpretation of the formalism we have established up to that
point.
In preparation for a more detailed discussion
on the foundations of bit-string physics, we note here that the alternatives
ADJOIN and TICK correspond {\it precisely}
to the production of a virtual particle represented by a 3-leg ``Feynman'' diagram,
or ``3-event'', and to the scattering process represented by a 4-leg ``Feynman''
diagram, or ``4-event''respectively.
We have to use quotes around Feynman, because our
diagrams obey finite and discrete conservation laws consistent
with measurement accuracy. This whole subject will be more
fully developed elsewhere, for instance in discussing
bit-string scattering theory \cite{75}.
Another aspect of program universe is worth mentioning. We note that
TICK has a {\it global} character since a 4-event anywhere in the
bit-string universe will necessarily produce a ``simultaneous''
increase of string length in our space of description. This means
that it will be a candidate for representing a coherent cosmological time in
an expanding universe model. The time interval to which TICK
refers is the shortest meaningful (i.e. finite and discrete)
distance that the radius of the universe can
advance in our theory divided by the velocity of light. We will return
to this idea on another occasion when we discuss cosmology.