SF is a special subject link-type used for certain "filler" subjects
like "it" and "there". It interacts heavily with
post-processing.  Post-processing is used both to enforce that
certain predicates may not be used with "filler" subjects, and
also that certain predicates may only be used with such
subjects. SF is also used with a few special phrases like "to"
and "that" phrases when used as subjects.

     Contents
     1. "Filler-it"
     2. Constraints on "filler-only" phrases
     3. "There" as a subject: SFst and SFp
     4. Special subjects: SFsx

"Filler-it"
Many verbs and adjectives take complements like "to+infinitive"
or "that+clause" (see "TO";
see "TH"):

	1. I expect that he will go
	2. I am glad that he is going
	3. He wants to go
	4. He is eager to go

However, there are certain adjective-complement and
verb-complement phrases that may only be used with the subject
"it".

	5. It is likely that Jane will go
	6. *Jane is likely that Jane will go
	7. It seems that Jane should go
        8. *Jane seems that Jane should go
	9. It was suggested that Jane would go
        10. *Jane was suggested that Jane would go
	11. It is important to go
        12. *Jane is important to go

It can be seen in the dictionary that the verbs and adjectives
on the left of "that" in exs. 5-10 - "seems", "likely" - have
specially subscripted THi+ connectors, unlike "expect" and
"glad" which have unsubscripted TH+ connectors. Moreover,
"wants" and "eager" have TO+ connectors; "important" has,
instead, a "TOi+" connector.  "It" also has a special "SFsi+"
connector; verbs which "It" may connect to directly are given
"SFs-" connectors, disjoined with their "S-". ("It" also has
an ordinary "Ss+" connector; see below). Thus the connectors
that may only be used with "it", as well as "it" itself, are
marked with special connectors. We use this information in
post-processing to make the distinctions noted above. 

Recall that post-processing divides the links of a sentence
into groups, corresponding roughly to subject-verb
expressions.  In post-processing, we insist that any group
which contains a THi link or a TOi link must contain an SFsi
link.  In this way, we are able to reject ex. 6 above while
accepting ex. 5.

	+SFsi+--Paf-+-THi+--Ce+-S(e)-+I(e)+
	|    |      |    |    |      |    |
	It  is   likely that Jane  will  go

	+-Ss-+--Paf-+-THi+--Ce+-S(e)-+I(e)+
	|    |      |    |    |      |    |
      *Jane  is   likely that Jane  will  go

Of course, "it" may also be used referentially, as an ordinary
pronoun: "It is black with a long tail". Thus "it" has the
following:

	it: SFsi+ or Ss+ or O- or J-;

A sentence like ex. 5 will thus get two parses, one with 
SFsi and one with Ss; the one with Ss will be rejected.	This
is not merely vanity, but has an important function, as
described below.

There are a number of other verb-complement situations where
"filler-it" is required as the subject. In all such cases,
the complement connector is given an "i" subscript, and
post-processing is used in the manner described above. These
include "Ci", used for embedded clauses without "that" ("It is
likely he'll do it"); "QIi", used in indirect questions ("It
is clear who did it"); CPi, Eqi, and COqi, used in
paraphrasing expressions ("She left, it seems"); TSi, used for
subjunctive clauses ("It is important that she go"; and O#i,
used in the construction "It is [object] who...".

Constraints on "Filler"-Only Phrases
Predicate phrases that may only be used with "it" as the
subject, such as "likely that" and "important to", might be
described as "filler-only".  There are other constraints on
the way such phrases are used, beside the fact that "it" must
be the subject. Only certain verbs and adjectives may be
combined with them:

	*It seems to be likely that Jane will go
	*It is glad to be likely that Jane will go

Thus, as well as enforcing that "it" is the subject of such
phrases, we must ensure that they are not used in combination
with predicates like "glad" and "wants to". Another way of
thinking about this is that there is a "filler-it" which may
not be used with "referential-only" predicates and a
referential-"it" which may not be used with "filler-only"
predicates. This is in fact the approach we use. The "S+"
connector on "it" corresponds to referential uses, the "SF+"
to "filler" uses.  We have already insisted that "filler-only"
predicates (like "likely that") are not used with "S"; we now
must require that "referential-only" phrases are not used with
"SF".  This is again enforced with post-processing rules. The
verb and adjective connectors (I, PP, Pg, Pa, Pv) on the verbs
and adjectives that may be used with "filler-it" ("be",
"seem") are subscripted with "f" (yielding If, PPf, Pgf, Paf,
Pvf); all others, like those on "want" and "glad", are left
unsubscripted. (Note that "it" can only make a direct SF
connection to certain verbs anyway; thus the use of
"filler-it" is partially controlled at the linkage level.)
Post-processing rules then dictate that when an SFsi is
present in a group, the only forms of verb connectors (I, PP,
Pg, Pa, and Pv) which are allowed are those subscripted with
an "f". The following linkage is thus rejected, because the
group containing an SFsi also contains unsubscripted "Pa" and
"TO" links.

        +SFsi+-Pa-+-TO+If+-Paf-+-THi-+--Ce+S(e)+I(e)+
        |    |    |   |  |     |     |    |    |    |
       *It  is  glad to  be likely that  Joe  will go

Many sentences with "it" receive two parses, one with "S" and
one with "SF". The above sentence, for example, also receives
this linkage:

        +-Ss+-Pa-+-TO+If+-Paf-+-THi-+--Ce+S(e)+I(e)+
        |   |    |   |  |     |     |    |    |    |
       *It is  glad to  be likely that  Joe  will go

This linkages is also rejected, however, because the "THi"
requires an "SFsi". If a sentence contains "filler"-only links
(in a group with "it"), the "S" parse will be invalid; if it
contains referential-only links, the "SF" parse will be
invalid. In this case, then, one linkage fails to meet the
"SF" requirements, the other fails to meet the "S"
requirements, and the sentence is rejected.

Note that the division of the sentence into domains is
essential here. Consider the sentence 

        +SFsi+-Paf+THi-+--Ce-+S(e)+If(e)+Pa(e)+TO(e)+I(e)+
        |    |    |    |     |    |     |     |     |    |
	It  is likely that Jane will    be  glad   to   go

If the sentence were not divided into domains, the
post-processing rules would see that there are unsubscripted
"Pa", "TO" and "I" links, which are incompatible with "SFsi"
(as well as a THi link, incompatible with the "S" usage of
"it"); thus the sentence would be rejected. With domains,
however, the parser knows that "THi" goes with "It" and "glad
to go" goes with "Jane", and everything is okay. The same
thing applies also, of course, with relative clauses,
subordinate clauses, and the like: "the weather was terrible,
but we thought it was likely that Jane, who was an excellent
sailor, would get the boat safely back to the shore". Such
constructions would obviously wreak havoc with post-processing
rules unless the clauses were clearly demarcated.

The same applies with sentences with "to":

        +SFsi+--Paf--+-TOi-+I(e)-+
        |    |       |     |     |
	it   is important to    go

In this case, the "TOi" on "important" must begin a new
domain, so that the post-processor knows that "go" does not
relate to "filler-it" (with which it is incompatible). The
tricky thing here is that "adjective+to" constructions do not
always start new domains: in sentences like "He is ready to
go", the infinitive clearly relates to the subject preceding
the adjective. For such adjectives, we use unsubscripted
TO. (Note that there are a few adjective-infinitive
constructions, such as "certain" and "likely", that do not
start new domains with "to", but which are compatible with
"filler-it": "It is certain to be important to go". Such
adjectives take "TOf".)

"There" as a subject: SFst and SFp	
SFst and SFp are used when "there" is acting as a subject:

          +-SFst-+
	  |      |
	There seems to be a problem.

          +-SFp-+
          |     |
	There seem to be problems.

"There" therefore has "SFst+ or SFp+"; verbs which can connect
directly to "there" have "SFst-" or "SFp-", as appropriate.
It may seem odd to distinguish between singular and plural
"there", since the forms are the same; this relates to
post-processing.

There are constraints on the way "there" can be used as a
subject, which are enforced mainly through post-processing.
This is similar to the case of "filler-it", only simpler. Like
"filler-it", "there" is compatible only with certain verbs and
adjectives:

	There seems to have been a problem
	There might appear to be a problem
	*There is eager to be a dog
	*There wants to try to be a dog

These are precisely the verbs and adjectives that are
compatible with "filler-it".  These verbs have already been
specially subscripted; post-processing rules enforce that
groups which contain unsubscripted links of these types
may not also contain a "SFsi" (used with "filler-it").  Thus
we simply make such links incompatible with "SFst" and
"SFp" as well. In this way, we prohibit incorrect sentences
like those above. (See "SF: Filler-it".)  Note, however, that,
unlike "it", "there" is always a "filler" subject; it
therefore has no ordinary "S+" connector.

There are other constraints on "there". The "filler-only"
phrases used with "it" (likely that, important to) may not be
used with "There": "*There is likely that Jane is
coming". Moreover, when "there" is acting as a subject, there
must be an object in the clause ("*There seems to be likely"),
and there must be number agreement between the object and the
verb ("*There seem to be a dog here"). This is again enforced
through post-processing. Forms of the verb "be" have O*t+
connectors. "O-" connectors on singular nouns (used in
ordinary direct-object links: see "O") are given "s"
subscripts; those on plural nouns have "p" subscripts.  When a
form of "be" connects to an Os- or Op- connector on a noun, an
Opt or Ost link is thus created:

          +-SFp-+            +-Ost-+
          |     |	     |     |
      *	There seem to have been a dog here

          +-SFst+             +-Ost-+
          |     |	      |     |
	There seems to have been a dog here

PP rules then dictate that every SFst must have an Ost in its
group and every SFp must have an Osp in its group. In the
process, we also ensure that phrases used with "filler-it",
like "likely that", do not occur with "there"; it turns out
that there is no way for a group to contain a direct object
and an predicative adjectival phrase like "likely that" at the
same time.

As discussed with "it", the use of post-processing domains is
important here. Some nouns take "to" phrases as complements:
"There was an effort to revive the bill".  A verb like
"revive" in this case is fine, because it does not relate to
the subject "there" ("*There seems to have revived the
bill"). But in order for the post-processing rules to know
this, the TOn link from the noun ("an EFFORT TO revive") must
begin a new group, letting the parser know that a new subject
is in force.

"Special Subjects": SFsx
SFsx is used for a few phrases that can act as subjects under
rather constrained circumstances: "that"-clause phrases,
"to"-infinitive phrases, and "where-when-why" phrases.

          +-------SFsx-----+
          +Ce-+-S--+       |
	  |   |    |       |
	That Joe  is angry is not surprising

	 +------SFsx------+
         +-I--+           |
	 |    |           |
	To invite Bill would be a mistake		

	  +------SFsx---+
	  +--Cs-+       |
          |     |       |
	Where they went is a mystery

In each case, the subject phrase is a kind of phrase that
occurs frequently as a verb complement. Thus we simply
directly disjoin and conjoin "SFsx+" on the head word with
whatever connectors are used for verb complements. With
"that", for example:

	that: Ce+ & (TH- or SFsx+)

As with gerunds, constructions of this kind seem to vary quite
gradually in their grammaticality (See "Ss#g").  However, they
are much rarer than gerunds, and seem much more constrained.

	Finishing college would make you more marketable
	?To finish college would make you more marketable
	The graduating of Fred changes the situation
	?That Fred graduated changes the situation
	
Most often, such special subject phrases are used with the
verb "be" and a few other verbs and adjectives ("seem",
"appear", "likely"). these are the same predicates that may be
taken by "filler-it" and "there". We already have a system in
place in post-processing for restricting the predicates used
with "filler-it" and "there" (which use "SFsi" and "SFst",
respectively). (See "SF: Filler-it".)  For now, then, we
simply apply those same constraints to "SFsx". (As with
"there", we allow forms of "be" to take direct objects under
these circumstances: "That John graduated is a problem".)
This solution could probably be improved, however.

There are further constraints on the use of "special
subjects". They may not occur in relative clauses (ex. 1), and
they may not invert with their auxiliary (exx. 2-3).

	*The problem that John graduated is is very large
	*Is that John graduated a problem?
	*Is to graduate a good idea?		

To solve the first problem, we give special subjects a
restricted "clause" expression conjoined with their "SF+"
connectors, omitting the "R-" connector which is conjoined
with S+ on ordinary nouns. To solve the second problem, we
simply do not give special subjects "SFI-" connectors.

SFsic is used with comparatives; see "MV: Comparatives III".