2. The argument in favor of the tripartite approach
cited instances of nesting. The second consideration is
is roughly the following: many proponents of formal
that the determination of external functioning by con-
grammar claim that it is possible to construct a single
text searching is not a simple one-shot operation. It is
simple parsing algorithm to be used with any of several
not always possible to formulate a particular single
grammars of a certain type. The type of grammar has
context for a particular sequence that is to be ex-
to be specified very precisely by means of a grammar-
amined. Rather, the variety of contextual conditions
rule format. These grammars can be written in the
which may apply to a particular construction may dif-
form of tables of rules, and the same algorithm can be
fer from sentence to sentence, and the particular con-
ditions that apply can be determined only by a gradu-
* Work on this paper was done under the sponsorship of the
ated search of a potentially ever extending range of
Information Processing Laboratory of the Rome Air Development
contexts. This means that one cannot simply talk of
Center of the United States Air Force, under Contract AF30(602)-
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