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From: blenko@rochester.UUCP (Tom Blenko)
Newsgroups: net.lang
Subject: Re: What's an Augmented Transition Network?
Message-ID: <5456@rochester.UUCP>
Date: Mon, 5-Mar-84 22:54:07 EST
Article-I.D.: rocheste.5456
Posted: Mon Mar  5 22:54:07 1984
Date-Received: Tue, 6-Mar-84 07:01:56 EST
References: <144@forcm5.UUCP>
Organization: U. of Rochester, CS Dept.
Lines: 45

Augmented transition networks (ATN's) are an extension of finite
state machine models for parsers. They were first applied by Bill
Woods and others at BBN for parsing of natural language.

A recursive transition networl (RTN) is just like a finite state
machine, except that "pushes" of subnetworks are permissible
transitions. Thus

S:
	 [NP]      [VP]
	----->SSub----->SPred

NP:
	 [Art]      [Noun]
	+------>Det+------->HN
	|	   |
	----------->

VP:
	 [Verb]	     [NP]
	-------->Mv+-------+>Vp
		   |       |
		   -------->

might represent a grammar in which a sentence (S) consists of a noun
phrase (NP) followed by a verb phrase (VP). The NP transition succeeds
if an optional article (Art) followed by a noun (Noun) can be read from
input. Similarly, the VP transition succeeds if a verb is read from
input, and an optional noun phrase can be "pushed".  Clearly an RTN is
no more powerful than a finite state parser, but it is easier to write
grammars for!

An ATN is an RTN extended to handle (virtually arbitrary) conditionals
attached to the transitions. This was intended to capture some of the
semantics (some would argue it's all syntax) of the language. For
example, a part of a grammar might apply to prepositional phrases for
"place where": these would check at the beginning of the phrase to make
sure that the preposition was one of {to, from, at}.

There is also a capability for storing values in registers on a
"push".  This is useful for handling relative clauses, for example, in
the which the noun modified in the main sentence might be used as
either the subject or object of the the relative clause.

	Tom