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In this new line, although there are four nodes, just one of them is a bud node,
whereas the remaining three are leaf nodes. From the leaf nodes obviously
no more growth will be sprouting, but from the bud node another branch will
be formed. Thus, the required branch is placed below the bud node and filled
with the next element in the ORF, the symbol “a” at position 10:
Q
b
a
Q
b
b
a
With this step, the expression of the K-expression (2.3) is complete as the last
line contains only nodes with terminals. We will see that, thanks to the structural
organization of GEP genes, the last line of all ETs generated by this technique
will contain only terminals. And this is equivalent to say that all programs
evolved by GEP are syntactically correct. Indeed, in GEP, there is no such
thing as an invalid expression or computer program.
Looking at the structure of ORFs only, it is difficult or even impossible to
see the advantages of such a representation, except perhaps for its simplicity
and elegance. However, when open reading frames are analyzed in the con-
text of a gene, the advantages of this representation become obvious. As I
said before, the chromosomes of gene expression programming have fixed
length, and they are composed of one or more genes of equal length. There-
fore the length of GEP genes is also fixed. Thus, in gene expression pro-
gramming, what varies is not the length of genes, which is constant, but the
length of the ORFs. Indeed, the length of an ORF may be equal to or less
than the length of the gene. In the first case, the termination point coincides
with the end of the gene, and in the latter the termination point is somewhere
upstream of the end of the gene. And this obviously means that GEP genes
have, most of the times, noncoding regions at their ends.
And what is the function of these noncoding regions at the end of GEP
genes? We will see that they are the essence of gene expression program-
