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PF is used in certain direct and indirect questions with "be". 
Normally forms of "be" require a complement to the right; but
they can also be satisfied by a question-word to the left like
"where", "when" or "how".

      |     |    |   |
    ///// Where are you

          +--QI-+    +-S-+
          |     |    |   |
     I wonder where you are

Forms of "be" thus have PF- directly disjoined with "O+ or Pg+
or Pv+...".

The question-words "where" and "when" have PF- conjoined with
Wq- or QI-. In questions, then, a Wq- connection to the wall
is made, enforcing s-v inversion (see "SI"); in indirect questions,
an QI connection is made, preventing s-v inversion.

Note that other verbs cannot be used in this way: "*Where ran
you", "*What think you", "*What have you?" The first two cases
are prevented because ordinary verbs don't have SI+
connectors; the third is prevented because, although "have"
has an SI+, it is conjoined only with PP+ (used in
past-participles) and not with O+ and B- (used with direct
objects). Only on forms of "be" is the SI+ conjoined with the
entire complement expression.

PF is also used in cases of fronted prepositional and participle
	  |                   |    |
	Among the candidates was Jane Smith, a professor
	Carrying the box     was a small child
	?Described by John   was a new program

Such phrases vary widely in their acceptability, depending on
the participle or preposition; we allow all participles and
most prepositions to be used this way, but only with a cost of
2.  (These phrases are not to be confused with openers.
Openers attach to the subject of an ordinary main clause,
which does not have s-v inversion and can have any verb:
"Carrying the box, Jane left". Fronted prepositional or
participle constructions involve s-v inversion; the phrase
must attach to a form of "be".) In such cases, the fronted
phrase must attach to the wall. S-v inversion is necessary
here ("*Among the candidates Jane Smith was"); thus the
participle makes a Wq connection to the wall, enforcing s-v
inversion in post-processing.

	among:    J+ & (Mp- or MVp- or ... [[Wq- & PF+]])
	carrying: (Pg- & (O+...))   or    
	    ((O+...) & (Ss*g+ or COp+ or [[Wq- & PF+]]));

For participles, note that the Wq- & PF+ complex is directly
disjoined with the COp+ and Ss*g+, and is to the right of the
complement expression; it is fully disjoined with the Pg-,
which is to the left of its complement expression.  See "CO:
Participles as Openers" for an explanation.

Grammar Documentation Page.