Unable to understand meaning of tag NONE *-1 in Penn Treebank example

Question

I am completely naive and new to NLP. I am following a course on NLP on coursera. As I was studying about parse tree, I came across following sentence to parse:

Because the CD had an effective yield of 13.4 % when it was issued in 1984 , and interest rates in general had declined sharply since then , part of the price Dr. Blumenfeld paid was a premium -- an additional amount on top of the CD 's base value plus accrued interest that represented the CD 's increased market value .

And partial parse tree to this sentence is:

(S
 (SBAR-PRP
   (IN Because)
    (S
      (S
        (NP-SBJ (DT the) (NNP CD))
        (VP
            (VBD had) (NP
            (NP (DT an) (JJ effective) (NN yield))
            (PP (IN of) (NP (CD 13.4) (NN %))))
                (SBAR-TMP
                (WHADVP-4 (WRB when))
                (S
                  (NP-SBJ-1 (PRP it))
                  (VP
                     (VBD was) (VP
                     (VBN issued)
                     (NP (-NONE- *-1))
                     (PP-TMP (IN in) (NP (CD 1984)))
                     (ADVP-TMP (-NONE- *T*-4))))))))

I am not able to get meaning of -NONE- T-4 and -NONE- *-1. Anyone can help me?

Answer 1

After going through a number of examples, I think I got meaning of *NONE* in parse tree output. Let's take an example, suppose we have to parse following sentence:

Ricky is a boy and likes fruits.

In above sentence two sentences are joined with coordinating conjunction and.
Sentence 1: Ricky is a boy.
Sentence 2: likes fruits.

In first sentence subject is clearly given which is Ricky but in second sentence it is not explicitly written (although it's clear we are talking about Ricky in second sentence too.). So when we parse such sentences, in the parse tree we get *NONE* for the NP (Noun Phrase).

Answer 2

Every leaf of the syntactic tree gets a Part-of-Speech (POS) label. When the leaf is an actual word, it is assigned a POS tag such as "N", "V", etc.

Some leaves are not actual words but instead trace elements. Traces represent placeholders in the syntax. All trace placeholders have "*" plus optional material. See treebank documentation for explanation of the types of traces. The "-4" above indicates the index for coreference.

In any case, the trace is assigned Part-of-Speech of "NONE" because it wouldn't make sense to assign a POS tag to a non-word.