PP connects forms of "have" with past participles:

            +-PP-+
	    |    |
	He has gone

Forms of "have" have "PP+" connectors. These are disjoined
with connectors for other uses of "have", such as O+ ("he has
a dog") and TO+ ("he has to go"). It is also conjoined with
"()", allowing "have" to be used with no complement: "She
hasn't seen it, but I have."  This is rare and carries a cost
of 2.

Past participles have "PP-" connectors, conjoined with their
complement connectors (O+, TH+, TO+, etc.). Since the past
participle form of the verb is usually the same as the simple
past form (which uses an "S-" connector), we can usually use
the same expression for both.

	died arrived moved purchased: (S- or PP-) & [complement];

In cases where the simple past and the past participle are
distinct, however, we must use separate expressions.

	began went forsook: S- & [complement];
	begun gone forsaken: PP- & [complement];

The past participle is, in every case, the same as the passive
form (where there is one); but the passive complement
expression is usually quite different from the past-participle
one, so the past-participle connector PP- and the passive
connector Pv- may not be directly disjoined. (See "Pv".)

PPf is used by post-processing to control the use of "it" and
"there"; see "SF: Filler-it".