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<Paper uid="E89-1001">
  <Title>Parsing Idioms in Lexicalized TAGs *</Title>
  <Section position="2" start_page="0" end_page="0" type="metho">
    <SectionTitle>
1 Introduction to Tree Ad-
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
joining Grammars
</SectionTitle>
      <Paragraph position="0"> Tree Adjoining Grammars (TAGs) were introduced by Joshi et al. 1975 and Joshi 1985 as a formalism for linguistic description. Their linguistic relevance was shown by Kroch and Joshi 1985 and Abeill@ 1988. A lexicalized version of the formalism was presented in Schabes, Abeill~ and Joshi 1988 that makes them attractive for writing computational grammars. They were proved to be  -1parsable in polynomial time (worst case) by Vijay Shanker and Joshi 1985 and an Earley-type parser was presented by Schabes and Joshi 1988.</Paragraph>
      <Paragraph position="1"> The basic component of a TAG is a finite set of elementary trees that have two types: initial trees or auxiliary trees (See Figure 1). Both are minimal (but complete) linguistic structures and have at least one terminal at their frontier (that is their 'head'). Auxiliary trees are also constrained to have exactly one leaf node labeled with a non-terminal of the same category as their root node.</Paragraph>
      <Paragraph position="3"> Sentences of the language of a TAG are derived from the composition of an S-rooted initial tree with elementary trees by two operations: substitution or adjunction.</Paragraph>
      <Paragraph position="4"> Substitution inserts an initial tree (or a tree derived from an initial tree) at a leaf node bearing the same label in an elementary tree (See Figure 2). 2 It is the operation used by CFGs.</Paragraph>
      <Paragraph position="5">  Adjunction is a more powerful operation: it inserts an auxiliary tree at one of the corresponding node of an elementary tree (See Figure 3).3 TAGs are more powerful than CFGs but only mildly so (Joshi 1983). Most of the linguistic advantages of the formalism come from the fact that it factors recursion from dependencies. Kroch and Joshi 1985 show how unbounded dependencies can be 'localized' by having filler and gap as part of 21 is the mark for substitution.</Paragraph>
      <Paragraph position="6"> SAt each node of an elementary tree, there is a feature structure associated with it (Vijayshanker and Joshi, 1988). Adjunction constraints can be defined in terms of feature structures and the success or failure of unification.</Paragraph>
      <Paragraph position="8"> the same elementary tree and having insertion of matrix clauses provided by recursive adjunctions.</Paragraph>
      <Paragraph position="9"> Another interesting property of the formalism is its extended domain of locality, as compared to that of usual phrase structure rules in CFG. This was used by Abeill~ 1988 to account for the properties of 'light' verb (often called 'support' verb for Romance languages) constructions with only one basic structure (instead of the double analysis or reanalysis usually proposed).</Paragraph>
      <Paragraph position="10"> We now define by an example the notion of derivation in a TAG.</Paragraph>
      <Paragraph position="11"> Take for example the derived tree in Figure 4.</Paragraph>
      <Paragraph position="12">  Unlike CFGs, from the tree obtained by deriva-2- null tion (called the derived tree) it is not always possible to know how it was constructed. The derivation tree is an object that specifies uniquely how a derived tree was constructed.</Paragraph>
      <Paragraph position="13"> The root of the derivation tree is labeled by an S-type initial tree. All other nodes in the derivation tree are labeled by auxiliary trees in the case of adjunction or initial trees in the case of substitution. A tree address is associated with each node (except the root node) in the derivation tree. This tree address is the address of the node in the parent tree to which the adjunction or substitution has been performed. We use the following convention: trees that are adjoined to their parent tree are linked by an unbroken line to their parent, and trees that are substituted are linked by dashed lines.</Paragraph>
      <Paragraph position="14"> The derivation tree in Figure 6 specifies how the derived tree was obtained:</Paragraph>
      <Paragraph position="16"> node of address 1, aNPdn\[man\] is substituted in the tree atnl\[saw\] at address 1, aNPn\[Mary\] is substituted in the tree atnl\[saw\] at node 2.2 and the tree \[3adS\[yesterday\] is adjoined in the tree atnl\[saw\] at node 0.</Paragraph>
      <Paragraph position="17"> In a 'lexicalized' TAG, the 'category' of each word in the lexicon is in fact the tree structure(s) it selects. 4 Elementary trees that can be linked by a syntactic or a lexical rule are gathered in a Tree Family, that is selected as a whole by the head of the structure. A novel parsing strategy follows (Schabes, Abeill~, :loshi 1988). In a first step, the parser scans the input string and selects the different tree structures associated with the lexical items of the string by looking up the lexicon. In a second step, these structures are combined together to produce a sentence. Thus the parser uses only a subset of the entire (lexicalized) grammar.</Paragraph>
    </Section>
  </Section>
  <Section position="3" start_page="0" end_page="0" type="metho">
    <SectionTitle>
2 Linguistic Properties of Id-
</SectionTitle>
    <Paragraph position="0"> ioms Idioms have been at stake in many linguistic discussions since the early transformational grammars, but no exhaustive work based on extensive listings of idioms have been pursued before Gross 1982. We rely on L.A.D.L.'s work for French that studied 8000 frozen sentences, 20, 000 frozen nouns and 6000 frozen adverbs. For English, we made use of Freckelton's thesis (1984) that listed more than 3000 sentential idioms. They show that, for a given structure, idiomatic phrases are usually more numerous in the language than 'free' ones. As is well known, idioms are made of the same lexicon and consist of the same sequences of categories as 'free' structures. An interesting exception is the case of 'words' existing only as part of an idiomatic phrase, such as escampette in prendre la poudre d'escampette (to leave furtively) or umbrage in to take umbrage at NP.</Paragraph>
    <Paragraph position="1"> The specificity of idioms is their semantic noncompositionality. The meaning of casser sa pipe (to die), cannot be derived from that of casser (to break) and that of pipe (pipe). They behave semantically as one predicate, and for example the whole VP casser sa pipe selects the subject of the sentence and all possible modifiers. We therefore consider an idiom as one entity in the lexicon.</Paragraph>
    <Paragraph position="2"> It would not make sense to have its parts listed in the lexicon as regular categories and to have special rules to limit their distribution to this unique context. If they are already listed in the lexicon, these existing entries are considered as mere homonyms. Furthermore, usually idioms are ambiguous between literal and idiomatic readings. null Idioms do not appear necessarily as contlnuous strings in texts. As shown by M. Gross for French and P. Freckelton for English, more than 15% of sentential idioms are made up of unbounded arguments, (e.g. NPo prendre NP1 en compte, NPo take NP1 into account, Butter would not melt in NP's mouth). Discontinuities can also come from the regular application of syntactic rules. For example, interposition of adverbs between verb and object in compound V-NP phrases, and interposition of modals or auxiliaries between subject and verb in compound NP-V phrases are very general (Laporte 1988).</Paragraph>
    <Paragraph position="3"> As shown by Gazdar et al. 1985 for English, and Gross 1982 for French, most sentential idioms are not completely frozen and 'transformations' apply to them much more regularly  -3than is usually thought. Freckelton 1984's listings of idiomatic sentences exhibit passivization for about 50% of the idioms comprised of a verb (different from be and have) and a frozen direct argument. Looking at a representative sample of 2000 idiomatic sentences with frozen objects (from Gross's listings at LADL) yields similar results for passivization and relativization of the frozen argument for French. This is usually considered a problem for parsing, since the order in which the frozen elements of an idiom appear might thus vary.</Paragraph>
    <Paragraph position="4"> Recognizing idioms is thus dependent on the whole syntactic analysis and it is not realistic to reanalyze them as simple categories in a preprocessing step.</Paragraph>
  </Section>
  <Section position="4" start_page="0" end_page="0" type="metho">
    <SectionTitle>
3 Representing Idioms in
</SectionTitle>
    <Paragraph position="0"> Lexicalized TAGs We represent idioms with the same elementary trees as 'free' structures. The values of the arguments of trees that correspond to a literal expression are introduced via syntactic categories and semantic features. However, the values of arguments of trees that correspond to an idiomatic expression are not only introduced via syntactic categories and semantic features but also directly specified.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.1 Extended Elementary Trees
</SectionTitle>
      <Paragraph position="0"> Some idioms select the same elementary tree structures as 'free' sentences. For example, a sentential idiom with a frozen subject il/aut S1 selects the same tree family as any verb taking a sentential complement (ex: NP0 dit $1), except that ii is directly attached in subject position, whereas a 'free' NP is inserted in NPo in the case of 'dit'  Usually idioms require elementary trees that are more expanded. Take now as another example the sentential idiom N Po kicked the bucket. The corresponding tree must be expanded up to the D1 and N1 level, the (resp. bucket) is directly attached to the D1 (resp. N1) node (See Figure 8).</Paragraph>
    </Section>
    <Section position="2" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.2 Multicomponent Heads
</SectionTitle>
      <Paragraph position="0"> In the lexicon, idiomatic trees are represented by specifying the elements of the idiom. An idiom as NPo kicked the bucket is indexed by a 'head' (kicked) which specifies the other pieces of the idiom. Although the idiom is indexed by one item, the pieces are considered as its multicomponent heads.5 We have, among others, the following entries in the lexicon: 6 kicked , V : Tnl (transitive verb) (a) kicked , V : Tdnl\[D1 = the, N1 = bucket\] (idiom) (b)  Suppose that the input sentence is John kicked the bucket. The first entry for kicked (a) specifies that kicked can be attached under the V node in the tree atdnl (See the tree c~tnl\[kicked\] in Figure 9). However the second entry for kicked (b) specifies that kicked can be attached under the V node and that the must be attached under the node labeled by D1 and that bucket must be attached under the node labeled N1 in the tree atnl (See the tree atdnl\[kicked-the-bucket\] in Figure 9).</Paragraph>
      <Paragraph position="1"> In the first pass, the trees in Figure 9 are be selected (among others).</Paragraph>
      <Paragraph position="2"> Some idioms allow some lexical variation, usually between a more familiar and a regular use of the same idiom, for example in French NPo per. dre la t~te and NPo perdre ia boule (to get mad). This is represented by allowing disjunction on the string that gets directly attached at a certain position in the idiomatic tree. NPo perdre ia t~te/boule will thus be one entry in the lexicon, and we do not have to specify that t~te and boule are synonymous (and restrict this synonymy to hold only for this context).</Paragraph>
    </Section>
    <Section position="3" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
3.3 Selection of Idiomatic Trees
</SectionTitle>
      <Paragraph position="0"> We now explain how the first pass of the parser is modified to select the appropriate possible candidates for idiomatic readings. Take the previous example, John kicked the bucket. The verb kicked will select the tree atdnl \[kicked-the-bucket\] for an idiomatic reading. However, the values of the determiner and the noun of the object noun phrase are imposed to be respectively the and bucket. The determiner and the noun are attached to the tree atdnl\[kicked-the-bucket\], however the tree atdnl\[kicked-the-bucket\] is selected if the words kicked, the and bucket appear in the input string at position compatible with the tree atrial\[kicked-the-bucket\]. Therefore they must respectively appear in the input string at some position i, j and k such that i &lt; j &lt; k. If it is not the case, the tree atdnl\[kicked-the-bucket\] is not selected. This process is called lexical attachment. null For example the word kicked in the following sentences will select the idiomatic tree atdn 1 \[kicked-the-bucket\]: John kicked the bucket (sl) John kicked the proverbial bucket (sP) John kicked the man who was carrying the bucket (s3) The parser will accept sentences sl and sP as idiomatic reading but not the sentence s3 since the tree atdnl\[kicked-the-bucket\] will fail in the parse. In the following sentence the word kicked will not select the idiomatic tree atdnl\[kicked-the-bucket\]: John kicked Mark (s4) John kicked a bucket (sS) John who was carrying a bucket kicked the child (s6) What did John kick? (sT) This test cuts down the number of idiomatic trees that are given to the parser as possible candidates. Thus a lot of idioms are ruled out before starting the syntactic analysis because we know all the lexical items at the end of the first pass. This is important because a given item (e.g. a verb) can be the head of a large number of idioms (Gross 82 has listed more than 50 of them for the verb manger, and prendre or avoir yield thousands of them). However, as sentence s3 illustrates, the test is not sufficient.</Paragraph>
      <Paragraph position="1"> What TAGs allow us to do is to define multicomponent heads for idiomatic structures without requiring their being contiguous in the input string. The formalism also allows us to access directly the different elements of the compound without flattening the structure. As opposed to CFGs, for example, direct dependencies can be expressed between arguments that are at different levels of depth in the tree without having to pass features across local domains. For example, in NPo rider DET sac (to express all of one's se-</Paragraph>
      <Paragraph position="3"> cret thoughts), the determiner of the object sac has to be a possessive and agree in person with the subject : je vide mon sac, tu rides ton sac...</Paragraph>
      <Paragraph position="4"> In NPo dire DET quatre veritds a NP2 (to tell someone what he really is), the determiner of the object veritds has to be a possessive and agree in person with the second object NP2 : je te dis tes quatre veritds, je lui dis ses quatre verit~s.</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="0" end_page="0" type="metho">
    <SectionTitle>
4 Literal and Idiomatic
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
Readings
</SectionTitle>
      <Paragraph position="0"> Our representation expresses correctly that idioms are semantically non-compositional. Trees obtained by lexical attachment of several lexical items act as one syntactic unit and also one semantic unit.</Paragraph>
      <Paragraph position="1"> For example, the sentence John kicked the bucket can be parsed in two different ways. One derivation is built with the trees: atnl\[kicked\] (transitive verb), aNPn\[John\], aD\[the\] and aNPn\[bucket\] . It corresponds to the literal interpretation; the other derivation is built with the trees: atdnl\[kicked-the-bucket\] (idiomatic tree) and aNPn\[John\] (John):</Paragraph>
      <Paragraph position="3"> However, both derivations have the same de- null The meaning of kicked the bucket in its idiomatic reading cannot be derived from that of kicked and the bucket. However, by allowing arguments to be inserted by substitution or adjunction (in for example atdnl \[kicked-the-bucket\]), we represent the fact that NPo kicked the bucket acts as a syntactic and semantic unit expecting one argument NPo.</Paragraph>
      <Paragraph position="4"> Similarly, NPo kicked NP1 in atnl\[kicked\] acts as a syntactic and semantic unit expecting two arguments NPo and NP1. This fact is reflected in the two derivation trees of John kicked the bucket.</Paragraph>
      <Paragraph position="5"> However, the sentential idiom 'il fant $1', is not parsed as ambiguous, since faut has only one entry (that is idiomatic) in the lexicon. When a certain item does not exist except in a specific idiom, for example umbrage in English, the corresponding idiom to take umbrage of NP will not be parsed as ambiguous. The same holds when a item selects a construction only in an idiomatic expression. Aller, for example, takes an obligatory PP (or adverbial) argument in its non-idiomatic sense. Thus the idiom: aller son train (to follow one's way) is not parsed as ambiguous since there is no free NPo aller NP1 structure in the lexicon.</Paragraph>
      <Paragraph position="6"> We also have ambiguities for compound nominals such as carte bleue, meaning either credit card (idiomatic) or blue card (literal), and for compound adverbials like on a dime: John stopped on a dime will mean either that he stopped in a controlled way or on a 10 cent coin.</Paragraph>
      <Paragraph position="7"> Structures for literal and idiomatic readings are both selected by the parser in the first step. Since syntax and semantics are processed at the same time, the sentence is analyzed as ambiguous between literal and idiomatic interpretations. The derived trees are the same but the derivation trees are different. For example, the adjective bleue selects an auxiliary tree that is adjoined to carte in the literal derivation tree, whereas it is directly attached in a complex initial tree in the case of idiomatic interpretation.</Paragraph>
      <Paragraph position="8"> All frozen elements of the idiom are directly attached in the corresponding elementary trees, and do not have to exist in the lexicon. They are thus distinguished from 'free' arguments that select their own trees (and their own semantics) to be substituted in a standard sentential tree.</Paragraph>
      <Paragraph position="9"> Therefore we distinguish two kinds of semantic operations: substitution (or adjunction) corresponds to a compositional semantics; direct attachment, on the other hand, makes different items behave as one semantic unit.</Paragraph>
      <Paragraph position="10"> One should notice that non-idiomatic readings are not necessarily literal readings. Since feature structures are used for selectional restrictions of arguments, metaphoric readings can be taken into account (Bishop, Cote and Abeill~ 1989).</Paragraph>
      <Paragraph position="11"> We are able to handle different kinds of semantic non-compositionality, and we do not treat as idiomatic all cases of non-literal readings.</Paragraph>
    </Section>
  </Section>
  <Section position="6" start_page="0" end_page="0" type="metho">
    <SectionTitle>
5 Recognizing
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
Discontinuous Idioms
</SectionTitle>
      <Paragraph position="0"> Parsing flexible idioms has received only partial solutions so far (Stock 1987, Laporte 1988). Since TAGs factor recursion from dependencies, discontinuities are captured straightforwardly without special devices (as opposed to Johnson 1985 or Bunt et al. 1987). We distinguish two kinds of discontinuities: discontinuities that come from internal structures and discontinuities that come from the insertion of modifiers.</Paragraph>
    </Section>
    <Section position="2" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
5.1 Internal Discontinuities
</SectionTitle>
      <Paragraph position="0"> Some idioms are internally discontinuous. Take for example the idioms NPo prendre NP1 en compte and NPo takes NP1 into account (see Figure 10). s The discontinuity is handled simply by arguments (here NPo and NP1) to be substituted (or adjoined in some cases) as any free sentences.</Paragraph>
      <Paragraph position="1"> The internal structures of arguments can be unbounded. null</Paragraph>
    </Section>
    <Section position="3" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
5.2 Recursive Insertions of Modi-
</SectionTitle>
      <Paragraph position="0"> fiers Some adjunctions of modifiers may be ruled out in idioms or some new ones may be valid only in idioms. If the sentence is possibly ambiguous between idiomatic and literal reading, the adjunction of such modifiers force the literal interpretation. For example, in NPo casser sa pipe (to die) , the NP1 node in the idiomatic tree bears a null adjunction constraint (NA). The sentence H a cassd sa pipe en bois (he broke his wooden pipe) is SNA expresses the fact that the node has null adjunction constraint then parsed as non-idiomatic. This NA constraint will be the only difference between the two derived trees (See Figure 11): Jean a cass~ sa pipe (literal) and Jean a cassg sa pipe (idiomatic).</Paragraph>
      <Paragraph position="1"> But most idioms allow modifiers to be inserted in them. Each modifier can be unbounded (e.g.</Paragraph>
      <Paragraph position="2"> with embedded adjunct clauses) and their insertion is recursive. We treat these insertion by adjunction of modifiers in the idiomatic tree. However constraint of adjunction and feature structure constraints filter out partially or totally the insertion of modifiers at each node of an idiomatic tree. In a TAG, the internal structure of idioms is specified in terms of a tree, and we can get a unified representation for such compound adverbials as la limite and ~ l' extreme limite (if there is no other way) or such complex determiners as a bunch of (or ia majoritd de NP ) and a whole bunch of NP (resp. la grande majoritd de NP) that will not have to be listed as separate entries in the lexicon. The adjective whole (resp. grande) adjoins to the noun bunch (resp. majoritd ), as to any noun. Take a bunch of NP. The adjective whole adjoins to the noun bunch as to any noun (See Figure 12) and builds a whole bunch of.</Paragraph>
      <Paragraph position="3"> In order to have a modifier with the right features adjoining at a certain node in the idiom, we associate some features with the head of the idiom (as for heads of 'free' structures) but also with elements of the idiom that are directly attached.</Paragraph>
      <Paragraph position="4"> Unification equations, such as those constraining agreement, are the same for trees selected by idioms and trees selected by 'free' structures. Thus only grande that is feminine singular, and not grand for example, can adjoin to majorit~ that is feminine singular. In il falloir NP, the frozen subject il is marked 3rd person singular, and only an auxiliary like va (that is 3rd person singular) and not vont (3rd person plural) will be allowed  to adjoin to the VP: il va falloir $1 and not il vont falloir $1.</Paragraph>
      <Paragraph position="5"> As another example, an idiom such as la moutarde monte au nez de NP (NP looses his temper) can be represented as contiguous in the elementary tree. Adjunction takes place at any internal node without breaking the semantic unity of the idiom. For example, an adjunct clause headed by anssit6t can adjoin between the frozen subject and the rest of the the idiom in la moutarde monter au nez de NP2 : la montarde, aussitSt que Marie enlra, monta an nez de Max (Max, as soon as Marie got in, lost his temper). Similarly, auxiliaries adjoin between frozen subjects and verbs as they do to 'free' VPs: There might have been a boz on the table is parsed as being derived from the idiom : there be NP1 P NP2.</Paragraph>
      <Paragraph position="6"> It should be noted that when a modifier adjoins to an interior node of an idiom, there is a semantic composition between the semantics of the modifier and that of the idiom as a whole, no matter at which interior node the adjunction takes place.</Paragraph>
      <Paragraph position="7"> For example, in John kicked the proverbial bucket semantic composition happens between the 3 units John, kick-the-bucket, and proverbial. 9 Semantic composition will be done the same way if an adjunct clause were adjoined into the VP. In John kicked the bucket, as the proverb says, composition will happen between John, kick-the.bucket, and the adjunct clause considered as one predicate as-proverb-say: 9This is the case of a modifier where adjoining is valid only for the idiom.</Paragraph>
      <Paragraph position="8"> Therefore parsing flexible idioms is reduced to the general parsing of TAGs (Schabes and Joshi 1988).</Paragraph>
    </Section>
  </Section>
  <Section position="7" start_page="0" end_page="0" type="metho">
    <SectionTitle>
6 Tree Families and Appli-
</SectionTitle>
    <Paragraph position="0"> cation of 'Transformations' to Idioms As in the case of predicates in lexicalized TAGs, sentential idioms are represented as selecting a set of elementary trees and not only one tree. These tree families gather all elementary trees that are possible syntactic realizations of a given argument structure. The family for transitive verbs, for example, is comprised of trees for wh-question on the subject, wh-question on the object, relativization on the subject, relativization on the object, and so on. In the first pass, the parser loads all the trees in the tree family corresponding to an item in the input string (unless certain trees in that family do not match with the feature of the head in the input string).</Paragraph>
    <Paragraph position="1"> The same tree families are used with idioms.</Paragraph>
    <Paragraph position="2"> However some trees in a family might be ruled out by an idiom if it does not satisfy one of the three following requirements.</Paragraph>
    <Paragraph position="3"> First, the tree must have slots in which the pieces of the idiom can be attached. Ideg If one distinguishes syntactic rules that keep the lexical value of an argument in a sentence (e.g. topicalization, cleft extraction, relativization...), and syntactic rules that do not (deleting the node for that argument, or replacing it by a pronoun or a whelement; e.g.: wh-question, pronominalization), it can be shown that usually only the former applies to frozen elements of an idiom. If you take the idiom bruler nn fen (to run a (red) light), relativization and cleft extraction, but not wh-question, are possible on the noun fen, with the idiomatic reading: null Le fen que Jean a brulg.</Paragraph>
    <Paragraph position="4"> C'est nn fen que Jean a brulg.</Paragraph>
    <Paragraph position="5"> * Que brule Jean ? Second, if all the pieces of an idiom can be attached in a tree, the order imposed by the tree must match with the order in which the pieces appear in the input string. Thus, if enfant appears before attendre in the input string, the hypothesis for an idiomatic reading will be made but only the trees corresponding to relativization, cleft exlOTllis requirement is independent of the input string.  -8traction, topicalization in which enfant is required to appear before attendre will be selected. But if the string enfant is not present at all ih the input string, the idiomatic reading will not be hypothesized, and trees corresponding to qui attend-elle will never be selected as part of the family of the idiom attendre nn enfant.</Paragraph>
    <Paragraph position="6"> Third, the features of the heads of an idiom must unify with those imposed on the tree (as for 'free' sentences). For example, it has to be specified that bncket in to kick the bucket does not undergo relativization nor passivization, whereas tabs in to keep tabs on NP does. It is well known that even for 'free' sentences application of the passive, for example, has somehow to be specified for each transitive verbs since there are lexical idiosyncrasies, aa The semantics of the passive tabs were kept on NP by NP is exactly the same as that of the active NP keep tabs on NP, since different trees in the same tree families are considered as (semantically) synonymous.</Paragraph>
  </Section>
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