*** Guide-to-Links ***
X
X is used to connect punctuation symbols to words. Xc connects
a word to a comma to the right; Xd connects a word to a comma
to a left. Xp and Xi are used for periods; Xx is used with
colons and semi-colons. In this entry we provide a general
discussion of punctuation.

     Contents
     1. Comma phrases
     2. Comma phrases at the beginning and end of sentences
     3. Comma phrases in sequence
     4. Xi: Abbreviations
     5. Xp: Periods at ends of sentences
     6. Xx and Xe: Colons and semi-colons
     7. Other punctuation symbols

Comma Phrases
A wide variety of words and phrases can be used with commas on
either side.  Some are noun-modifiers, using MX to connect to
a previous noun. Some are verb-modifiers, using MV to connect
to a previous verb or E to connect to a following one. Some
are "openers", using CO to connect to a following clause
subject. And some are quoting expressions ("he said"), which
can be inserted at various places in the sentence. In each of
these cases, the head word of the expression must link to
commas on either side.

          +-MX---+   
          |  +Xd-+-----Xc-------+
          |  |   |              |
1.	John , a doctor		, is here	 
2.	John , with his mother  , is here
3.	John , carrying the dog , left the room
4.	John , who you know     , is here
5.	John ,  to whom I spoke , is here

          +--MV--+
          |  +Xd-+-----Xc-------+
          |  |   |              |		
6.   We left , carrying the dog , and Fred followed
7.   We left , quietly	        , and Fred followed     
8.   We left , with Jane	, and Fred followed
9.   We left , when we saw him  , and Fred followed

	             +-------CO-------+
                 +Xd-+-----Xc-------+ |
                 |   |              | | 	     
10. He said that , after the party  , he had gone home
11. He said that , eventually	    , he had gone home
12. He said that , after he saw us  , he had gone home

                    +---Xd-+--Xc-------+
                    |      |           |
    	    He left , he said	       , and Joe followed   (CP-)
	       Jane , he said	       , left the party     (Eq+)
14. After the party , he said	       , he left	    (COq+)

In each case, the head word must connect most closely to
the words within the comma expression; then to the commas
themselves; then to any words outside. So, for example, in
ex. 2 above, prepositions have

with:	J+ & (.... or (Xc+ & Xd- & MX-))

	^  	       ^      ^     ^
 link to prep.    links to commas  link to previous 
  object			   noun

In some cases, the commas are obligatory (such as noun
modifiers of nouns, like ex. 1: "*Jane a doctor is here"). In
other cases the commas are essentially optional, but for
various reasons, we decided to use a different link-types for
with-comma and without-comma expressions (for example,
prepositional phrases modifying nouns use M without commas,
MX with commas). In other cases, the commas are optional and
the same connector is used with or without; this is the case,
for example, with manner adverbs modifying verbs. Such adverbs
therefore take "{Xc+ & Xd-} & MVa-".

One special case must be discussed. On opener phrases, the
following comma is optional; the preceding comma is also
optional, and is only permitted when there is a following
comma. 

	He said that after the party he went home
	He said that after the party, he went home
	He said that, after the party, he went home
	*He said that, after the party he went home

Openers therefore take "{{Xd-} & Xc+} & CO+".

Comma Phrases at the Beginning and End of Sentences
It was said that, with the exception of openers, expressions
generally require either two commas or none; it is not
permitted to have only a comma at the beginning or only one at
the end.  One very important exception to all this must be
mentioned, namely, comma expressions at the beginnings and
endings of sentences. The case of phrases at the beginning has
been dealt with, since these are always openers; there the
beginning comma may be omitted.  With phrases at the end,
however, the linkage expressions proposed above will require a
comma at the end of the sentence, and this is clearly wrong. A
comma is not only not required; it is incorrect. (The same
applies to openers; the expressions provided will allow a
comma at the very beginning of the sentence.) As described
above, then, the system will accept the incorrect sentences 1
and 2 below, and reject the correct sentence 3.

		 +-Xd+-----Xc-------+
		 |   |              |
1.	*He left , carrying the dog ,
2.	*        , After the party  , he left
3.	He left  , carrying the dog

Regarding the false positives 1 and 2, we simply allow these.
Rules of the form "word X is not permitted at the beginning
or end of a sentence" do not seem to arise in syntax generally,
only in punctuation; therefore we have not considered it
important to accommodate them. (Clearly, a system for weeding
out sentences beginning or ending with commas could easily be
devised.) What is important is to allow sentence 3 above. This
we do by installing a "right-hand wall". The right-hand wall has
an "Xc-" connector; this may satisfy the demand of a comma-phrase
head-word for a following comma, when the phrase occurs at the
end of the sentence. Ex. 3 above will therefore be accepted:

		 +-Xd+------Xc--------+
		 |   |                |
	 He left , carrying the dog /////

This punctuation usage is the only function of the right-hand
wall. In cases where the punctuation function is not needed,
we give the right-hand wall an "RW-", and the left-hand wall
an "RW+"; thus the two walls connect to each other, preserving
connectivity.

Comma Phrases in Sequence
In many cases, comma phrases can be used in sequence. These
can either be cases of one comma phrase nested inside another
(ex. 1 below), or one comma phrase following another, both
modifying the same previous word (ex.2) or the same following
word (ex.3); or one comma phrase modifying a previous word,
and another modifying a following word (ex.4).

	        +--------------------Xc----+
      +------MV-+         +---MX-----+     |
      |   +-Xd--+         |  +---Xd--+-Xc+ |
      |   |     |         |  |       |   | |
1.He left , carrying the dog ,  a poodle , , and Fred followed

	  +---------MX--------+
	  +---MX---+	      |
          |  +-Xd--+-Xc-+  +Xd+---Xc-----+
          |  |     |    |  |  |          |
2.	Jane , a doctor ,  , who you met , is here

3.	After he saw Jane, Fred said, he left the party
4. 	Although I liked my doctor, Mr. Smith, later, 
                I chose a different one

The structures shown for exx. 1 and 2 are those that would be
required given the expressions above. However, this is
obviously wrong.  Each phrase requires a comma before and
afterwards; but multiple commas in a row are not required,
indeed (again) they are not permitted. What seems to happen is
that a single comma fulfills the demands of more than one
phrase. In ex. 1, the final comma fulfills the Xc demand of
both phrases; in ex. 2, the comma in the middle fulfills the
Xc demand of the previous phrase and the Xd demand of the
following one. We handle this by giving the comma the
following expression:

	",": {@Xc-} & (Xc- or Xd+)

Now, as well as making an obligatory Xd or Xd connection, it
can make any number of additional Xc connections as well.
For exs. 1 and 2 above, then, this yields:

		      +----------------Xc-------+
	    +------MV-+         +---MX---+      |
	    |   +-Xd--+         |  +-Xd--+--Xc--+
            |   |     |         |  |     |      |
	He left , carrying the dog , a poodle   , and Fred followed

	  +---------MX-----+
	  +---MX---+	   |
          |  +-Xd--+-Xc-+Xd+---Xc-----+
          |  |     |    |  |          | 
	Jane , a doctor , who you met , is here

There are three problems here. First, when a comma is serving
the demands for more than one phrase, it is creating a cycle;
it is linking two phrases together that are indirectly linked
in some other way. Cycles are dangerous; they may allow
non-cyclic expressions to form where they are not wanted
(for example, "Jane, a doctor I saw Fred, a doctor, is here").
To prevent these, we modify the comma expression slightly:

	",": ({@Xca-} & (Xd+ or Xc-))

In post-processing, we then add "Xca" to the
"MUST_FORM_A_CYCLE" list. This means that that any time an Xca
connection is made (i.e., any time a single comma is being
used in more than one phrase), the Xca must be part of a
cycle.

It is clear that the function of fulfilling several Xc+
demands may also be performed by the right-hand wall: "We
left, carrying the dog, a poodle". Thus the right-hand wall is
given not just Xc-, but "{@Xca-} & Xc-".

A second problem involves post-processing. The groups of
links formed in post-processing are supposed to correspond to
clauses. In some cases it seems appropriate for comma
phrases to form groups; for example, in relative clauses
("Jane , who you met , is here"). Again, however, a problem
arises with multiple comma phrases. Consider the following
sentence. Here, if left unchecked, the domain started by the
first MX link will spread via the second comma to the
second comma phrase ("a doctor"), which is clearly not part
of the relative clause, but rather a direct modifier of the
main noun. What's worse, it will then spread back through
the second MX link and then through the rest of the
sentence.

	  +-----------------S(r)------------+
	  +-----------MX(r)--------+	    |
	  |     +---Xca(r)--+      |	    |
	  +MX(r)+--B(r)--+  |      |        |
	  |  +Xd+  +S(r)-+  +Xd(r)-+Xc(r)+  |
	  |  |  |  |     |  |      |     |  |
	Jane , who you  met , a doctor   , is here

We solve this problem by creating an "IGNORE_THESE_LINKS" list
in post-processing. Links in this list are simply ignore by
post-processing; that is, no domains are traced through
them. We add "Xca" to this list. Then, in the above sentence,
the "r" domain started by the first MX will not spread through
the Xca to the second comma phrase; it will be confined to the
relative clause.

A third problem involves false positives. Return to the cases
discussed above, involving consecutive commas: "Jane , who you
met , , a doctor , is here". We have allowed for the correct
version of this sentence, in which the functions of the
consecutive commas are all served by a single comma. But we
have not prohibited the consecutive-comma sentence, which is
of course incorrect. Here again, we have not worried much
about this false positive, since the kind of rule involved
("Do not allow more than one X in a row") seems to arise only
in punctuation; presumably it could easily be dealt with if it
were important to do so.

Xi: Abbreviations
Xi is used to connect periods to previous strings that need
abbreviations. These include titles (Sen., Rev.), terms for
streets and institutions (Ave., Co.), month abbreviations, and
single letters, which are often used in names.

Xp: Periods at ends of sentences
Xp simply connects the period to the left wall at the end
of a sentence. It is optional; sentences do not have to end
in periods. In cases where a period is used, the period, not
the left-wall, makes an RW connection to the right wall. 
Exclamation marks and question marks also have Xp-, and thus
can also end sentences. We do not distinguish between these
symbols; any of the three may be used with any sentence.

Xx and Xe: Colons and Semi-Colons
Xx is used with colons and semi-colons. Unlike comma phrases -
where a word is usually the head of the phrase, with the comma(s)
acting as appendanges - with colons and semi-colons, the two
phrases are attached together through the punctuation symbol.
More specifically, the "Xx" attaches the colon or semi-colon back 
to the wall.

  +-----Xx-----+        
  +-Wd--+      +Wd+
  |     |      |  |
/////	I left ; Joe stayed

  +----------Xx--------+        
  +-Wd--+              +Wd+
  |     |              |  |              
/////	I have an idea : we should go
		         that John should go
	                 plastics

With semi-colons, the following phrase must be a clause;
semi-colons thus carry "Xx- & W+". With colons, the following
phrase can be a clause, "to-" or "that-" phrase, noun
phrase, or question; thus colons have "Xx- & (W+ or J+ or TH+
or TOn+ or Qd+)".

Colons may also be used in quoting expressions. For this 
purpose, we give colons Xe-, and give quoting verbs Xe+ & Ce+.

            +--Ce--+
            +Xe-+  +
            |   |  |
	He said : "I refuse to comment."

Other Punctuation Symbols
Other symbols besides commas may be used with "comma phrases":
parentheses "()" and double-dashes  "--". These symbols 
carry Xd- and Xc+, and may therefore be used in all the kinds
of comma phrases listed above.

	Jane Smith (a doctor) attended the meeting
        Jane Smith -- a doctor -- attended the meeting
        We left (carrying the dog) and Fred followed

Again, there are false positives here. Parenthese may be used
anywhere commas are used, but this is sometimes incorrect:
"Today) Jane left". Also, when a symbol is used on one end of
a commma expression, there is no guarantee that the compatible
symbol will be used on the other end: "Jane (a doctor, is
here."

Double dashes are also sometimes used like colons, and are
followed by noun-phrases, clauses, etc.. For this we give "--"
the same expression described above for ":".

As noted above, "?" and "!" may be used at the ends of sentences,
like periods. One difference is that they may also be used at
the end of a quoted expression (followed by a quoting
expression), whereas periods may not. 

        "What are you doing?" he asked.
        *"I am doing this." she replied.

For this purpose we give "?" and "!" Xq+. Quoting verbs such
as "said" are given Xq-, and may thus hook back to a "?" or
"!" rather than the usual comma.

"$" and "%" must be used with numbers. "$" must make an NI
connection forward to a number; "%" must make an ND connection
backwards. These symbols may then be used as noun phrases: "We
lost 50% of our money", "I have $5". An ampersand "&" may connect
two proper noun words together; thus it carries "G- & G+".

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