6   Null elements

This section describes the null elements which are assigned nodes the annotation scheme. Note that gaps for controlled subject positions (controlees ---see section 26), gaps formed by Across the Board extraction (ATB --- (see section 27), and gaps caused by right-node raising (see section 30.2) and N-bar deletion (see 30.1) are not assigned nodes in the annotation.

6.1   Null elements without indexing

Null elements include trace markers of relative clauses, null expletives, and zero pronouns of various kinds. None of these carry indexing information. A list of unindexed null elements used is shown in Table 1 along with specifications for their use.

null element	meaning
*T*	relative clause trace
*arb*	zero pronoun with generic impersonal reference
*exp*	null expletive
*pro*	zero pronoun with discourse reference (small pro)
*hearer*	zero pronoun with hearer referent
*speaker*	zero pronoun with speaker referent
*speaker+hearer*	zero pronoun with speaker and hearer referent
*speaker+pro*	zero pronoun with speaker and discourse referent

Table 1: List of null elements without indexing

     As shown in the following example, *arb* is used for generic impersonal reference.

(12)
論文を書くのはとてもたいへんです。
‘It's very tough to write a research paper.’

( (IP-MAT (PP (NP (IP-EMB (NP-SBJ *pro*)
                          (PP (NP (N 論文))
                              (P を))
                          (NP-OB1 *を*)
                          (VB 書く))
                  (N の))
              (P は))
          (NP-SBJ *)
          (ADVP (ADV とても))
          (ADJN たいへん)
          (AX です)
          (PU 。))
  (ID 2_misc_EXAMPLE))

     Sentences that appear not to have subjects are supplied with null element subject (NP-SBJ *exp*). This includes:

• weather predicates: 今日は吹雪いている/時雨れている/時化ている/春めいている/暑い/寒い, etc.

• sentences formed entirely on modal/evidential formal nouns: 鍵が開いているはずだ/わけだ/ものだ, etc.

• and other types of sentences formed entirely on nominal predicates.

The following sentence is an example of this last type:

(13)
……道理で返事をしないはずだ、

( (IP-MAT (NP-SBJ *exp*)
          (PU ……)
          (PP (NP (N 道理))
              (P で))
          (NP-PRD (IP-EMB (NP-SBJ *pro*)
                          (PP (NP (N 返事))
                              (P を))
                          (NP-OB1 *を*)
                          (VB し)
                          (NEG ない))
                  (N はず))
          (AX だ)
          (PU 、))
  (ID 200_aozora_Togawa-1937))

If an utterance can be construed as a complete assertion about a state of affairs in the world, a clausal analysis is favored and an expletive subject is supplied where an interpretable subject is not retrievable from context:

  (CP-EXL (IP-SUB (NP-SBJ *exp*)
                  (NP-PRD (N 火事))
                  (AX だ))
          (PU !))

     Like an overt pronoun, *pro* may refer to an entity in the discourse context, or may take as an antecedent an element in the same sentence. *hearer*, *speaker*, *speaker+hearer* and *speaker+pro* are more specified versions of *pro*, and are preferred over *pro* when appropriate.

(14)
津波だと思いました。

( (IP-MAT (NP-SBJ *speaker*)
          (CP-THT (IP-SUB (NP-SBJ *pro*)
                          (NP-PRD (N 津波))
                          (AX だ))
                  (P と))
          (VB 思い)
          (AX まし)
          (AXD た)
          (PU 。))
  (ID 91_news_KAHOKU_34))

(15)
「先生、もうお忘れですか？」

( (CP-QUE (-LRB- 「)
          (IP-SUB (NP-VOC (N 先生))
                  (PU 、)
                  (NP-SBJ *hearer*)
                  (ADVP (ADV もう))
                  (VB お忘れ)
                  (AX です))
          (P か)
          (PU ？)
          (-RRB- 」))
  (ID 92_aozora_Hayashida-2015))

     In examples (12)--(15), zero pronouns mark non-controlled covertly expressed core grammatical roles (NP-SBJ, NP-OB1), i.e., arguments that are obligatory for interpreting the clausal predicate, but which have null expression and which cannot be reconstructed from either control relations (see section 26) or antecedent relations arising from coordination structures (see section 27). Again, the obligatory status of arguments compels the assumption of null pronouns where arguments are not overtly expressed.

     A trace *T* indicates the existence of an empty position in a relative clause modifying a head N that corresponds to that position. For more details, see section 22.1. Example (16) includes a trace for a subject (NP-SBJ *T*) to link the head (N 人) with that core grammatical role in the relative clause.

(16)
わかった人は手をあげてごらんなさい。」

( (IP-IMP (PP (NP (IP-REL (NP-SBJ *T*)
                          (VB わかっ)
                          (AXD た))
                  (N 人))
              (P は))
          (NP-SBJ *)
          (PP (NP (N 手))
              (P を))
          (NP-OB1 *を*)
          (VB あげ)
          (P て)
          (VB2 ごらん)
          (VB0 なさい)
          (PU 。)
          (-RRB- 」))
  (ID 137_aozora_Miyazawa-1934))

     Note that traces are not limited to core grammatical roles: Traces can indicate the presence of “gaps” in adjunct positions as well as in argument position. In this sense traces contrast with zero pronouns like (NP-SBJ *pro*), which correspond only to recoverable arguments. For details, see section 7.2.3.

     For an IP-REL that doesn't contain a coordinated clause, when it modifies a given N₁, there can only be one local trace corresponding to N₁. For an IP-REL that does contain a coordinated clause, there can be at most one trace per conjunct, although there is no constraint on the grammatical functions of these traces, and no requirement that all the conjuncts contain a trace. Note how in (17) the first conjunct in the coordinated IP-REL has a trace in object positions, while the second conjunct has a trace in subject position:

(17)
彼女は皆が愛しそして皆を愛する人です。

( (IP-MAT (PP (NP (PRO 彼女))
              (P は))
          (NP-SBJ *)
          (NP-PRD (IP-REL (IP-ADV (NP-OB1 *T*)
                                  (PP (NP (Q 皆))
                                      (P が))
                                  (NP-SBJ *が*)
                                  (VB 愛し))
                          (CONJ そして)
                          (NP-SBJ *T*)
                          (PP (NP (Q 皆))
                              (P を))
                          (NP-OB1 *を*)
                          (VB 愛する))
                  (N 人))
          (AX です)
          (PU 。))
  (ID 28_misc_EXAMPLE))

The second trace in (17) is placed to the right of the first conjunct. Otherwise, Across the Board extraction would require co-referential positions with identical grammatical roles in both conjuncts.

     One more thing to note about traces: If a an overt pronoun or *pro* in a relative clause is co-referent with the trace *T*, this relationship must be indicated by adding binding information not on the trace, but on the NP that the modified N projects:

(18)
図書館が市民に貸し出して戻ってこない本がたくさんある．

( (IP-MAT (PP (NP (IP-REL (NP-SBJ *T*)
                          (IP-ADV (NP-OB1 *pro*)
                                  (PP (NP (N 図書館))
                                      (P が))
                                  (NP-SBJ *が*)
                                  (PP (NP (N 市民))
                                      (P に))
                                  (VB 貸し出し)
                                  (P て))
                          (SCON *)
                          (VB 戻っ)
                          (P て)
                          (VB2 こ)
                          (NEG ない))
                  (N 本))
              (P が))
          (NP-SBJ *が*)
          (NP;*SBJ* (Q たくさん))
          (VB ある)
          (PU ．))
  (ID 747_dict_vv-lexicon))

6.2   Null elements that are always indexed

While the null elements of the previous section have no indexing, there is one further null element that must be indexed:

null element	meaning
*ICH*	abbreviation mnemonic for “interpret constituent here”

Table 2: Null element with indexing

*ICH* (abbreviating “interpret constituent here”) is used as a trace marking discontinuous structures, e.g., rightmost “afterthought” (see section 33.4, long-distance scrambling, or other remote relationships that cross phrase levels but are neither calculated (by control, ATB, relative clause extraction), nor specified by binding information (see section 6.3 for discussion). Indexing works by having an index number added to the label of the relocated constituent and incorporated into *ICH* to indicate where the remote constituent should be interpreted. For example, the following remote particle phrase corresponds to an indexed *ICH* trace. Note that disambiguation information (e.g., (NP-SBJ *)) is placed after the instance of *ICH* and not after the remote element.

(19)
美しい街ですよ、神戸は。
‘Kobe is a beautiful town.’

( (CP-FINAL (IP-SUB (PP *ICH*-1)
                    (NP-SBJ *)
                    (NP-PRD (IP-REL (NP-SBJ *T*)
                                    (ADJI 美しい))
                            (N 街))
                    (AX です))
            (P よ)
            (PU 、)
            (PP-1 (NP (NPR 神戸))
                  (P は))
            (PU 。))
  (ID 989_textbook_kisonihongo))

     It is possible for a single element to be coindexed with multiple instances of *ICH*, as in examples (20) and (21):

(20)
2012年末の男性平均寿命80.18歳、女性平均寿命84.67歳、戸籍人口平均寿命82.41歳。

( (IP-MAT (PP-1 (NP (NUMCLP (NUM 2012)
                            (CL 年))
                    (N 末))
                (P の))
          (IP-ADV (IP-ADV (NP-SBJ (PP *ICH*-1)
                                  (N 男性平均寿命))
                          (NP-PRD (NUMCLP (NUM 80.18)
                                          (CL 歳)))
                          (AX *))
                  (CONJ *)
                  (PU 、)
                  (NP-SBJ (PP *ICH*-1)
                          (N 女性平均寿命))
                  (NP-PRD (NUMCLP (NUM 84.67)
                                  (CL 歳)))
                  (AX *))
          (CONJ *)
          (PU 、)
          (NP-SBJ (PP *ICH*-1)
                  (N 戸籍人口平均寿命))
          (NP-PRD (NUMCLP (NUM 82.41)
                          (CL 歳)))
          (AX *)
          (PU 。))
  (ID 53_wikipedia_Shanghai))

(21)
何のわざとらしさも不自然さもなかった。

( (IP-MAT (NP-SBJ (PP-1 (NP (WPRO 何))
                        (P の))
                  (CONJP (NP (PP *ICH*-1)
                             (N わざとらしさ))
                         (P も))
                  (NP (PP *ICH*-1)
                      (N 不自然さ))
                  (P も))
          (ADJI なかっ)
          (AXD た)
          (PU 。))
  (ID 120_wikipedia_Audrey_Hepburn))

     Note that cases in which constituents may have been reordered leftward but remain at the same clause level (e.g., topicalisation, short-distance scrambling, etc.) are not indicated.

(22)
この服は太った人でも着られます。
‘Even a fat person can wear these clothes.’

( (IP-MAT (PP (NP (D この)
                  (N 服))
              (P は))
          (NP-OB1 *)
          (PP (NP (IP-REL (NP-SBJ *T*)
                          (VB 太っ)
                          (AX た))
                  (N 人))
              (P でも))
          (NP-SBJ *)
          (VB 着)
          (VB2 られ)
          (AX ます)
          (PU 。))
  (ID 859_textbook_kisonihongo))

By contrast a rightmost “afterthought” (see section 33.4 is placed at a different level from the clause which should contain a coindexed *ICH*, thereby ensuring the main predicate has rightmost placement:

(23)
「見よ、神の小羊」。

( (IP-IMP (-LRB- 「)
          (NP-SBJ *hearer*)
          (NP-OB1 *ICH*-1)
          (VB 見よ)
          (PU 、)
          (NP-1 (PP (NP (N 神))
                    (P の))
                (N 小羊))
          (-RRB- 」)
          (PU 。))
  (ID 63_bible_new))

6.3   The position of null elements

With null elements not being visible, string placement for a null element is often not obvious. Position is therefore determined as follows:

• Almost all relative clause traces (*T*) appear in constituent-initial position (but observe remarks on trace placement given later in this section).
• An expletive (*exp*) is IP-initial, with the exception that a trace is placed before an expletive.
• In general, zero pronouns (*arb*, *pro*, etc.) are placed IP-initial, so before conjunctions, and left-dislocated elements, but after traces (*T*) and expletives (*exp*) (but observe the influence of antecedent calculation discussed in the remainder of this section).

(24)
このビルは、現在の耐震基準に照らし合わせると、強度が不足している．

( (IP-MAT (PP (NP (D この)
                  (N ビル))
              (P は))
          (NP-SBJ *)
          (PU 、)
          (PP (IP-ADV (NP-SBJ *exp*)
                      (NP-OB1 *pro*)
                      (PP (NP (PP (NP (N 現在))
                                  (P の))
                              (N 耐震基準))
                          (P に))
                      (VB 照らし合わせる))
              (P と))
          (SCON *)
          (PU 、)
          (PP (NP (N 強度))
              (P が))
          (NP-SBJ2 *が*)
          (VB 不足)
          (VB0 し)
          (P て)
          (VB2 いる)
          (PU ．))
  (ID 1818_dict_vv-lexicon))

(25)
よそから、もらったお酒が二升あった。

( (IP-MAT (PP (NP (IP-REL (NP-OB1 *T*)
                          (NP-SBJ *speaker*)
                          (PP (NP (N よそ))
                              (P から))
                          (PU 、)
                          (VB2 もらっ)
                          (AXD た))
                  (N お酒))
              (P が))
          (NP-SBJ *が*)
          (NP;*SBJ* (NUMCLP (NUM 二)
                            (CL 升)))
          (VB あっ)
          (AXD た)
          (PU 。))
  (ID 66_aozora_Dazai-1-1940))

     The calculation of antecedents for empty positions can affect the placement of zero pronouns. For example, the calculation observes a rule for co-reference into coordinate clauses called “Across the Board” extraction (ATB). Specifically, any overt category x in IP_n must be an antecedent to one empty position (preserving its grammatical role) in each of coordinate clauses IP_{1, 2...n-1} disambiguated with CONJ provided that x is sister to and precedes IP_n-1, and provided that IP_n-2 is the first element in IP_n-1, and provided that IP_n-3 is the first element in IP_n-2, and so on. This requires that each of IP_{1, 2...n-1} “inherit” the antecedent relation from the category in IP_n. (See section 27.3, where the stacking structure of multiple clausal conjuncts with CONJ is diagramatically illustrated.) Placement of a zero pronoun in IP_n may be decided by whether that zero pronoun is an antecedent for all non-final conjuncts (in which case the zero pronoun is leftmost):

(26)
信じられないようなことですが超常現象ではありません

( (IP-MAT (NP-SBJ *pro*)
          (PP (IP-ADV (NP-PRD (IP-REL (NP-SBJ *T*)
                                      (NP-PRD (IP-EMB (NP-SBJ *pro*)
                                                      (VB 信じ)
                                                      (VB2 られ)
                                                      (NEG ない))
                                              (N よう))
                                      (AX な))
                              (N こと))
                      (AX です))
              (P が))
          (CONJ *)
          (NP-PRD (N 超常現象))
          (AX で)
          (P は)
          (VB2 あり)
          (AX ませ)
          (NEG ん))
  (ID 11_ted_talk_2))

or by whether the zero pronoun is a constituent of just one conjunct (in which case the zero pronoun is placed together with the other constituents of that conjunct):

(27)
喧嘩の呼吸で来られては見当が付かず扱かいにくい。

( (IP-MAT (NP-SBJ *speaker*)
          (IP-ADV (PP (IP-ADV (NP-OB1 *pro*)
                              (IP-SMC (PP (NP (PP (NP (N 喧嘩))
                                                  (P の))
                                              (N 呼吸))
                                          (P で))
                                      (VB 来))
                              (VB られ)
                              (P て))
                      (P は))
                  (CND *)
                  (PP (NP (N 見当))
                      (P が))
                  (NP-OB1 *が*)
                  (VB 付か)
                  (NEG ず))
          (CONJ *)
          (NP-OB1 *pro*)
          (VB 扱かい)
          (AX にくい)
          (PU 。))
  (ID 117_aozora_Kunieda-1925))

     Sometimes a control relation that would be established by following the default practice of placing zero pronouns in clause-initial position must be prevented. This is sometimes made necessary by the antecedent accessibility hierarchy, a part of the calculation of antecedents for co-reference relations which stipulates that controlled positions are more accessible by object-antecedents than by subject-antecedents. Given the antecedent accessibility hierarchy, subject-antecedent control into a subordinate clause requires that no object appear to the left of that clause. See an example of this scenario below. A clause IP₁ headed by a transitive verb but not containing an overt NP-OB1 argument, shares a subject with a subordiante clause IP₂ , but does not share an object. The more accessible NP-OB1 argument cannot appear to the left of IP₂, lest it bind into IP₂.

(28)
女の肩に手を掛けて、引き起して、窓の方へ向けて見ると、まだ二十にならない位な、すばらしい別品だったと云うのだ。」

( (IP-MAT (NP-SBJ *exp*)
          (CP-THT (IP-SUB (PP (IP-ADV (NP-SBJ *pro*)
                                      (IP-ADV (IP-ADV (IP-ADV (PP (NP (PP (NP (N 女))
                                                                          (P の))
                                                                      (N 肩))
                                                                  (P に))
                                                              (PP (NP (N 手))
                                                                  (P を))
                                                              (NP-OB1 *を*)
                                                              (VB 掛け)
                                                              (P て))
                                                      (CONJ *)
                                                      (PU 、)
                                                      (NP-OB1 *pro*)
                                                      (VB 引き起し)
                                                      (P て))
                                              (CONJ *)
                                              (PU 、)
                                              (NP-OB1 *pro*)
                                              (PP (NP (PP (NP (N 窓))
                                                          (P の))
                                                      (N 方))
                                                  (P へ))
                                              (VB 向け)
                                              (P て))
                                      (CONJ *)
                                      (VB 見る))
                              (P と))
                          (SCON *)
                          (PU 、)
                          (NP-SBJ *pro*)
                          (NP-PRD (IP-REL (NP-SBJ *T*)
                                          (NP-PRD (IP-EMB (ADVP (ADV まだ))
                                                          (PP (NP (NUMCLP (NUM 二十)))
                                                              (P に))
                                                          (NP-OB1 *に*)
                                                          (VB なら)
                                                          (NEG ない))
                                                  (N 位))
                                          (AX な))
                                  (PU 、)
                                  (IP-REL (NP-SBJ *T*)
                                          (ADJI すばらしい))
                                  (N 別品))
                          (AX だっ)
                          (AXD た))
                  (P と))
          (VB 云う)
          (FN の)
          (AX だ)
          (PU 。)
          (-RRB- 」))
  (ID 174_aozora_Mori-1912))

     For other configurations, there are fewer conditions on drawing co-reference relations. The clause marked with a conditional conjunctive particle たら in (29) contains an overt object NP that co-refers to the null subject of the main clause. Precedence is a sufficient condition for inferring the possibility of coreference here, and this motivates the non-initial placement of *pro*. Placing the zero pronoun (NP-SBJ *pro*) of the matrix to the right of the IP-ADV conditional clause allows the NP-OB1 argument of the preceding IP-ADV to be taken as antecedent for that zero pronoun, although without some extra information, coreference is only left open as a possibility, rather than being completely determined. In example (29), the NP-OB1 それを of predicate あげたら in the subordinate clause and the null pronoun subject of なくなった in the matrix clause (NP-SBJ *pro*) uniquely share “binding information”: (IP-MAT (IP-ADV (...)(PP (NP;{MIKAN} (PRO それ)) (P を)) (...) あげたら) (NP-SBJ;{MIKAN} *pro*) (...) なくなった)

(29)
それを三つ彼にあげたらなくなった。
‘When I gave him three (of them) they were all gone.’

( (IP-MAT (IP-ADV (NP-SBJ *speaker*)
                  (PP (NP (PRO それ))
                      (P を))
                  (NP-OB1 *を*)
                  (NP;*OB1* (NUMCLP (NUM 三)
                                    (CL つ)))
                  (PP (NP (PRO 彼))
                      (P に))
                  (NP-OB2 *に*)
                  (VB あげ)
                  (P たら))
          (CND *)
          (NP-SBJ *pro*)
          (VB なくなっ)
          (AXD た)
          (PU 。))
  (ID 35_misc_EXAMPLE))

     While co-reference between a downstairs null element and an overt antecedent to its right (“left pronominalization”) is also possible, that relationship can be calculated neither through precedence nor through the accessibility hierarchy. In such a scenario, binding information is absolutely necessary for establishing co-reference. This is demonstated by instances of (NP-OB1;{FILE} *pro*) in example (30) that are used to establish co-reference.

(30)
ダウンロードして印刷すれば、学校や家庭で手軽に取り組める。

( (IP-MAT (NP-SBJ *pro*)
          (IP-ADV (NP-OB1 *pro*)
                  (IP-ADV (VB ダウンロード)
                          (VB0 し)
                          (P て))
                  (CONJ *)
                  (VB 印刷)
                  (VB0 すれ)
                  (P ば))
          (CND *)
          (PU 、)
          (NP-OB1 *pro*)
          (PP (NP (CONJP (NP (N 学校))
                         (P や))
                  (NP (N 家庭)))
              (P で))
          (ADVP (ADJN 手軽)
                (AX に))
          (VB 取り組める)
          (PU 。))
  (ID 55_news_KAHOKU_97))

     There are annotation tools that will display the calculated anaphoric relations as indices, allowing annotators to spot mistakes in zero pronoun placement without having to refer directly to the antecedent accessibility hierarchy or to the various structural conditions for control and ATB antecedence.