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==Coverage==
The GF Resource Grammar Library contains grammar rules for
10 languages (in addition, 2 languages are available as incomplete
implementations, and a few more are under construction). Its purpose
is to make these rules available for application programmers,
who can thereby concentrate on the semantic and stylistic
aspects of their grammars, without having to think about
grammaticality. The targeted level of application grammarians
is that of a skilled programmer with
a practical knowledge of the target languages, but without
theoretical knowledge about their grammars.
Such a combination of
skills is typical of programmers who, for instance, want to localize
software to new languages.
The current resource languages are
- ``Ara``bic (incomplete)
- ``Cat``alan (incomplete)
- ``Dan``ish
- ``Eng``lish
- ``Fin``nish
- ``Fre``nch
- ``Ger``man
- ``Ita``lian
- ``Nor``wegian
- ``Rus``sian
- ``Spa``nish
- ``Swe``dish
The first three letters (``Eng`` etc) are used in grammar module names.
The incomplete Arabic and Catalan implementations are
enough to be used in many applications; they both contain, amoung other
things, complete inflectional morphology.
==A first example==
To give an example application, consider a system for steering
music playing devices by voice commands. In the application,
we may have a semantical category ``Kind``, examples
of ``Kind``s being ``Song`` and ``Artist``. In German, for instance, ``Song``
is linearized into the noun "Lied", but knowing this is not
enough to make the application work, because the noun must be
produced in both singular and plural, and in four different
cases. By using the resource grammar library, it is enough to
write
```
lin Song = mkN "Lied" "Lieder" neuter
```
and the eight forms are correctly generated. The resource grammar
library contains a complete set of inflectional paradigms (such as
``mkN`` here), enabling the definition of any lexical items.
The resource grammar library is not only about inflectional paradigms - it
also has syntax rules. The music player application
might also want to modify songs with properties, such as "American",
"old", "good". The German grammar for adjectival modifications is
particularly complex, because adjectives have to agree in gender,
number, and case, and also depend on what determiner is used
("ein amerikanisches Lied" vs. "das amerikanische Lied"). All this
variation is taken care of by the resource grammar function
```
mkCN : AP -> CN -> CN
```
(see the table in the end of this document for the list of all resource grammar
functions). The resource grammar implementation of the rule adding properties
to kinds is
```
lin PropKind kind prop = mkCN prop kind
```
given that
```
lincat Prop = AP
lincat Kind = CN
```
The resource library API is devided into language-specific
and language-independent parts. To put it roughly,
- the lexicon API is language-specific
- the syntax API is language-independent
Thus, to render the above example in French instead of German, we need to
pick a different linearization of ``Song``,
```
lin Song = mkN "chanson" feminine
```
But to linearize ``PropKind``, we can use the very same rule as in German.
The resource function ``mkCN`` has different implementations in the two
languages (e.g. a different word order in French),
but the application programmer need not care about the difference.
==Note on APIs==
From version 1.1 onwards, the resource library is available via two
APIs:
- original ``fun`` and ``oper`` definitions
- overloaded ``oper`` definitions
Introducing overloading in GF version 2.7 has been a success in improving
the accessibility of libraries. It has also created a layer of abstraction
between the writers and users of libraries, and thereby makes the library
easier to modify. We shall therefore use the overloaded API
in this document. The original function names are mainly interesting
for those who want to write or modify libraries.
==A complete example==
To summarize the example, and also give a template for a programmer to work on,
here is the complete implementation of a small system with songs and properties.
The abstract syntax defines a "domain ontology":
```
abstract Music = {
cat
Kind,
Property ;
fun
PropKind : Kind -> Property -> Kind ;
Song : Kind ;
American : Property ;
}
```
The concrete syntax is defined by a functor (parametrized module),
independently of language, by opening
two interfaces: the resource ``Syntax`` and an application lexicon.
```
incomplete concrete MusicI of Music =
open Syntax, MusicLex in {
lincat
Kind = CN ;
Property = AP ;
lin
PropKind k p = mkCN p k ;
Song = mkCN song_N ;
American = mkAP american_A ;
}
```
The application lexicon ``MusicLex`` has an abstract syntax that extends
the resource category system ``Cat``.
```
abstract MusicLex = Cat ** {
fun
song_N : N ;
american_A : A ;
}
```
Each language has its own concrete syntax, which opens the
inflectional paradigms module for that language:
```
concrete MusicLexGer of MusicLex =
CatGer ** open ParadigmsGer in {
lin
song_N = mkN "Lied" "Lieder" neuter ;
american_A = mkA "amerikanisch" ;
}
concrete MusicLexFre of MusicLex =
CatFre ** open ParadigmsFre in {
lin
song_N = mkN "chanson" feminine ;
american_A = mkA "américain" ;
}
```
The top-level ``Music`` grammars are obtained by
instantiating the two interfaces of ``MusicI``:
```
concrete MusicGer of Music = MusicI with
(Syntax = SyntaxGer),
(MusicLex = MusicLexGer) ;
concrete MusicFre of Music = MusicI with
(Syntax = SyntaxFre),
(MusicLex = MusicLexFre) ;
```
Both of these files can use the same ``path``, defined as
```
--# -path=.:present:prelude
```
The ``present`` category contains the compiled resources, restricted to
present tense; ``alltenses`` has the full resources.
To localize the music player system to a new language,
all that is needed is two modules,
one implementing ``MusicLex`` and the other
instantiating ``Music``. The latter is
completely trivial, whereas the former one involves the choice of correct
vocabulary and inflectional paradigms. For instance, Finnish is added as follows:
```
concrete MusicLexFin of MusicLex =
CatFin ** open ParadigmsFin in {
lin
song_N = mkN "kappale" ;
american_A = mkA "amerikkalainen" ;
}
concrete MusicFin of Music = MusicI with
(Syntax = SyntaxFin),
(MusicLex = MusicLexFin) ;
```
More work is of course needed if the language-independent linearizations in
MusicI are not satisfactory for some language. The resource grammar guarantees
that the linearizations are possible in all languages, in the sense of grammatical,
but they might of course be inadequate for stylistic reasons. Assume,
for the sake of argument, that adjectival modification does not sound good in
English, but that a relative clause would be preferrable. One can then use
restricted inheritance of the functor:
```
concrete MusicEng of Music =
MusicI - [PropKind]
with
(Syntax = SyntaxEng),
(MusicLex = MusicLexEng) **
open SyntaxEng in {
lin
PropKind k p = mkCN k (mkRS (mkRCl which_RP (mkVP p))) ;
}
```
The lexicon is as expected:
```
concrete MusicLexEng of MusicLex =
CatEng ** open ParadigmsEng in {
lin
song_N = mkN "song" ;
american_A = mkA "American" ;
}
```
==Lock fields==
//This section is only relevant as a guide to error messages that have to do with lock fields, and can be skipped otherwise.//
FIXME: this section may become obsolete.
When the categories of the resource grammar are used
in applications, a **lock field** is added to their linearization types.
The lock field for a category ``C`` is a record field
```
lock_C : {}
```
with the only possible value
```
lock_C = <>
```
The lock field carries no information, but its presence
makes the linearization type of ``C``
unique, so that categories
with the same implementation are not confused with each other.
(This is inspired by the ``newtype`` discipline in Haskell.)
For example, the lincats of adverbs and conjunctions are the same
in ``CatEng`` (and therefore in ``GrammarEng``, which inherits it):
```
lincat Adv = {s : Str} ;
lincat Conj = {s : Str} ;
```
But when these category symbols are used to denote their linearization
types in an application, these definitions are translated to
```
oper Adv : Type = {s : Str ; lock_Adv : {}} ;
oper Conj : Type = {s : Str} ; lock_Conj : {}} ;
```
In this way, the user of a resource grammar cannot confuse adverbs with
conjunctions. In other words, the lock fields force the type checker
to function as grammaticality checker.
When the resource grammar is ``open``ed in an application grammar,
and only functions from the resource are used in type-correct way, the
lock fields are never seen (except possibly in type error messages).
If an application grammarian has to write lock fields herself,
it is a sign that the guarantees given by the resource grammar
no longer hold. But since the resource may be incomplete, the
application grammarian may occasionally have to provide the dummy
values of lock fields (always ``<>``, the empty record).
Here is an example:
```
mkUtt : Str -> Utt ;
mkUtt s = {s = s ; lock_Utt = <>} ;
```
Currently, missing lock field produce warnings rather than errors,
but this behaviour of GF may change in future.
==Parsing with resource grammars?==
The intended use of the resource grammar is as a library for writing
application grammars. It is not designed for parsing e.g. newspaper text. There
are several reasons why this is not practical:
- Efficiency: the resource grammar uses complex data structures, in
particular, discontinuous constituents, which make parsing slow and the
parser size huge.
- Completeness: the resource grammar does not necessarily cover all rules
of the language - only enough many to be able to express everything
in one way or another.
- Lexicon: the resource grammar has a very small lexicon, only meant for test
purposes.
- Semantics: the resource grammar has very little semantic control, and may
accept strange input or deliver strange interpretations.
- Ambiguity: parsing in the resource grammar may return lots of results many
of which are implausible.
All of these problems should be solved in application grammars.
The task of resource grammars is just to take care of low-level linguistic
details such as inflection, agreement, and word order.
It is for the same reasons that resource grammars are not adequate for translation.
That the syntax API is implemented for different languages of course makes
it possible to translate via it - but there is no guarantee of translation
equivalence. Of course, the use of functor implementations such as ``MusicI``
above only extends to those cases where the syntax API does give translation
equivalence - but this must be seen as a limiting case, and bigger applications
will often use only restricted inheritance of ``MusicI``.
=To find rules in the resource grammar library=
==Inflection paradigms==
Inflection paradigms are defined separately for each language //L//
in the module ``Paradigms``//L//. To test them, the command
``cc`` (= ``compute_concrete``)
can be used:
```
> i -retain german/ParadigmsGer.gf
> cc mkN "Schlange"
{
s : Number => Case => Str = table Number {
Sg => table Case {
Nom => "Schlange" ;
Acc => "Schlange" ;
Dat => "Schlange" ;
Gen => "Schlange"
} ;
Pl => table Case {
Nom => "Schlangen" ;
Acc => "Schlangen" ;
Dat => "Schlangen" ;
Gen => "Schlangen"
}
} ;
g : Gender = Fem
}
```
For the sake of convenience, every language implements these five paradigms:
```
oper
mkN : Str -> N ; -- regular nouns
mkA : Str -> A : -- regular adjectives
mkV : Str -> V ; -- regular verbs
mkPN : Str -> PN ; -- regular proper names
mkV2 : V -> V2 ; -- direct transitive verbs
```
It is often possible to initialize a lexicon by just using these functions,
and later revise it by using the more involved paradigms. For instance, in
German we cannot use ``mkN "Lied"`` for ``Song``, because the result would be a
Masculine noun with the plural form ``"Liede"``.
The individual ``Paradigms`` modules
tell what cases are covered by the regular heuristics.
As a limiting case, one could even initialize the lexicon for a new language
by copying the English (or some other already existing) lexicon. This would
produce language with correct grammar but with content words directly borrowed from
English - maybe not so strange in certain technical domains.
==Syntax rules==
Syntax rules should be looked for in the module ``Constructors``.
Below this top-level module exposing overloaded constructors,
there are around 10 abstract modules, each defining constructors for
a group of one or more related categories. For instance, the module
``Noun`` defines how to construct common nouns, noun phrases, and determiners.
But these special modules are seldom or never needed by the users of the library.
TODO: when are they needed?
Browsing the libraries is helped by the gfdoc-generated HTML pages,
whose LaTeX versions are included in the present document.
==Special-purpose APIs==
To give an analogy with the well-known type setting software, GF can be compared
with TeX and the resource grammar library with LaTeX.
Just like TeX frees the author
from thinking about low-level problems of page layout, so GF frees the grammarian
from writing parsing and generation algorithms. But quite a lot of knowledge of
//how// to write grammars is still needed, and the resource grammar library helps
GF grammarians in a way similar to how the LaTeX macro package helps TeX authors.
But even LaTeX is often too detailed and low-level, and users are encouraged to
develop their own macro packages. The same applies to GF resource grammars:
the application grammarian might not need all the choices that the resource
provides, but would prefer less writing and higher-level programming.
To this end, application grammarians may want to write their own views on the
resource grammar.
==Browsing by the parser==
A method alternative to browsing library documentation is
to use the parser.
Even though parsing is not an intended end-user application
of resource grammars, it is a useful technique for application grammarians
to browse the library. To find out which resource function implements
a particular structure, one can just parse a string that exemplifies this
structure. For instance, to find out how sentences are built using
transitive verbs, write
```
> i english/LangEng.gf
> p -cat=Cl "she loves him"
PredVP (UsePron she_Pron) (ComplV2 love_V2 (UsePron he_Pron))
```
The parser returns original constructors, not overloaded ones. Overloaded
constructors can be returned, so far with experimental heuristics, by using
the grammar ``api/toplevel/OverLangEng.gf`` and a special flag:
```
> i api/toplevel/OverLangEng.gf
> p -cat=Cl -overload "she loves him"
mkCl (mkNP she_Pron) love_V2 (mkNP he_Pron)
```
Parsing with the English resource grammar has an acceptable speed, but
with most languages it takes just too much resources even to build the
parser. However, examples parsed in one language can always be linearized into
other languages:
```
> i italian/LangIta.gf
> l PredVP (UsePron she_Pron) (ComplV2 love_V2 (UsePron he_Pron))
lo ama
```
Therefore, one can use the English parser to write an Italian grammar, and also
to write a language-independent (incomplete) grammar. One can also parse strings
that are bizarre in English but the intended way of expression in another language.
For instance, the phrase for "I am hungry" in Italian is literally "I have hunger".
This can be built by parsing "I have beer" in ``OverLangEng`` and then writing
```
lin IamHungry =
let beer_N = mkN "fame" feminine
in
mkCl (mkNP i_Pron) have_V2 (mkNP massQuant beer_N)
```
which uses ``ParadigmsIta.mkN``.
==Example-based grammar writing==
The technique of parsing with the resource grammar can be used in GF source files,
endowed with the suffix ``.gfe`` ("GF examples"). The suffix tells GF to preprocess
the file by replacing all expressions of the form
```
in Module.Cat "example string"
```
by the syntax trees obtained by parsing "example string" in ``Cat`` in ``Module``.
For instance,
```
lin IamHungry =
let beer_N = mkN "fame" feminine
in
(in LangEng.Cl "I have beer") ;
```
will result in the rule displayed in the previous section. The normal binding rules
of functional programming (and GF) guarantee that local bindings of identifiers
take precedence over constants of the same forms. Thus it is also possible to
linearize functions taking arguments in this way:
```
lin
PropKind car_N old_A = in LangEng.CN "old car" ;
```
However, the technique of example-based grammar writing has some limitations:
- Ambiguity. If a string has several parses, the first one is returned, and
it may not be the intended one. The other parses are shown in a comment, from
where they must/can be picked manually.
- Lexicality. The arguments of a function must be atomic identifiers, and are thus
not available for categories that have no lexical items.
For instance, the ``PropKind`` rule above gives the result
```
lin
PropKind car_N old_A = AdjCN (UseN car_N) (PositA old_A) ;
```
However, it is possible to write a special lexicon that gives atomic rules for
all those categories that can be used as arguments, for instance,
```
fun
cat_CN : CN ;
old_AP : AP ;
```
and then use this lexicon instead of the standard one included in ``Lang``.

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==Texts. phrases, and utterances==
The outermost linguistic structure is ``Text``. ``Text``s are composed
from Phrases (``Phr``) followed by punctuation marks - either of ".", "?" or
"!" (with their proper variants in Spanish and Arabic). Here is an
example of a ``Text`` string.
```
John walks. Why? He doesn't want to sleep!
```
Phrases are mostly built from Utterances (``Utt``), which in turn are
declarative sentences, questions, or imperatives - but there
are also "one-word utterances" consisting of noun phrases
or other subsentential phrases. Some Phrases are atomic,
for instance "yes" and "no". Here are some examples of Phrases.
```
yes
come on, John
but John walks
give me the stick please
don't you know that he is sleeping
a glass of wine
a glass of wine please
```
There is no connection between the punctuation marks and the
types of utterances. This reflects the fact that the punctuation
mark in a real text is selected as a function of the speech act
rather than the grammatical form of an utterance. The following
text is thus well-formed.
```
John walks. John walks? John walks!
```
What is the difference between Phrase and Utterance? Just technical:
a Phrase is an Utterance with an optional leading conjunction ("but")
and an optional tailing vocative ("John", "please").
==Sentences and clauses==
TODO: use overloaded operations in the examples.
The richest of the categories below Utterance is ``S``, Sentence. A Sentence
is formed from a Clause (``Cl``), by fixing its Tense, Anteriority, and Polarity.
For example, each of the following strings has a distinct syntax tree
in the category Sentence:
```
John walks
John doesn't walk
John walked
John didn't walk
John has walked
John hasn't walked
John will walk
John won't walk
...
```
whereas in the category Clause all of them are just different forms of
the same tree.
The difference between Sentence and Clause is thus also rather technical.
It may not correspond exactly to any standard usage of the terms
"clause" and "sentence".
Figure 1 shows a type-annotated syntax tree of the Text "John walks."
and gives an overview of the structural levels.
#BFIG
```
Node Constructor Value type Other constructors
-----------------------------------------------------------
1. TFullStop Text TQuestMark
2. (PhrUtt Phr
3. NoPConj PConj but_PConj
4. (UttS Utt UttQS
5. (UseCl S UseQCl
6. TPres Tense TPast
7. ASimul Anter AAnter
8. PPos Pol PNeg
9. (PredVP Cl
10. (UsePN NP UsePron, DetCN
11. john_PN) PN mary_PN
12. (UseV VP ComplV2, ComplV3
13. walk_V)))) V sleep_V
14. NoVoc) Voc please_Voc
15. TEmpty Text
```
#BCENTER
Figure 1. Type-annotated syntax tree of the Text "John walks."
#ECENTER
#EFIG
Here are some examples of the results of changing constructors.
```
1. TFullStop -> TQuestMark John walks?
3. NoPConj -> but_PConj But John walks.
6. TPres -> TPast John walked.
7. ASimul -> AAnter John has walked.
8. PPos -> PNeg John doesn't walk.
11. john_PN -> mary_PN Mary walks.
13. walk_V -> sleep_V John sleeps.
14. NoVoc -> please_Voc John sleeps please.
```
All constructors cannot of course be changed so freely, because the
resulting tree would not remain well-typed. Here are some changes involving
many constructors:
```
4- 5. UttS (UseCl ...) ->
UttQS (UseQCl (... QuestCl ...)) Does John walk?
10-11. UsePN john_PN ->
UsePron we_Pron We walk.
12-13. UseV walk_V ->
ComplV2 love_V2 this_NP John loves this.
```
==Parts of sentences==
The linguistic phenomena mostly discussed in both traditional grammars and modern
syntax belong to the level of Clauses, that is, lines 9-13, and occasionally
to Sentences, lines 5-13. At this level, the major categories are
``NP`` (Noun Phrase) and ``VP`` (Verb Phrase). A Clause typically
consists of just an ``NP`` and a ``VP``.
The internal structure of both ``NP`` and ``VP`` can be very complex,
and these categories are mutually recursive: not only can a ``VP``
contain an ``NP``,
```
[VP loves [NP Mary]]
```
but also an ``NP`` can contain a ``VP``
```
[NP every man [RS who [VP walks]]]
```
(a labelled bracketing like this is of course just a rough approximation of
a GF syntax tree, but still a useful device of exposition).
Most of the resource modules thus define functions that are used inside
NPs and VPs. Here is a brief overview:
**Noun**. How to construct NPs. The main three mechanisms
for constructing NPs are
- from proper names: "John"
- from pronouns: "we"
- from common nouns by determiners: "this man"
The ``Noun`` module also defines the construction of common nouns.
The most frequent ways are
- lexical noun items: "man"
- adjectival modification: "old man"
- relative clause modification: "man who sleeps"
- application of relational nouns: "successor of the number"
**Verb**.
How to construct VPs. The main mechanism is verbs with their arguments,
for instance,
- one-place verbs: "walks"
- two-place verbs: "loves Mary"
- three-place verbs: "gives her a kiss"
- sentence-complement verbs: "says that it is cold"
- VP-complement verbs: "wants to give her a kiss"
A special verb is the copula, "be" in English but not even realized
by a verb in all languages.
A copula can take different kinds of complement:
- an adjectival phrase: "(John is) old"
- an adverb: "(John is) here"
- a noun phrase: "(John is) a man"
**Adjective**.
How to constuct ``AP``s. The main ways are
- positive forms of adjectives: "old"
- comparative forms with object of comparison: "older than John"
**Adverb**.
How to construct ``Adv``s. The main ways are
- from adjectives: "slowly"
- as prepositional phrases: "in the car"
==Modules and their names==
This section is not necessary for users of the library.
TODO: explain the overloaded API.
The resource modules are named after the kind of
phrases that are constructed in them,
and they can be roughly classified by the "level" or "size" of expressions that are
formed in them:
- Larger than sentence: ``Text``, ``Phrase``
- Same level as sentence: ``Sentence``, ``Question``, ``Relative``
- Parts of sentence: ``Adjective``, ``Adverb``, ``Noun``, ``Verb``
- Cross-cut (coordination): ``Conjunction``
Because of mutual recursion such as in embedded sentences, this classification is
not a complete order. However, no mutual dependence is needed between the
modules themselves - they can all be compiled separately. This is due
to the module ``Cat``, which defines the type system common to the other modules.
For instance, the types ``NP`` and ``VP`` are defined in ``Cat``,
and the module ``Verb`` only
needs to know what is given in ``Cat``, not what is given in ``Noun``. To implement
a rule such as
```
Verb.ComplV2 : V2 -> NP -> VP
```
it is enough to know the linearization type of ``NP``
(as well as those of ``V2`` and ``VP``, all
given in ``Cat``). It is not necessary to know what
ways there are to build ``NP``s (given in ``Noun``), since all these ways must
conform to the linearization type defined in ``Cat``. Thus the format of
category-specific modules is as follows:
```
abstract Adjective = Cat ** {...}
abstract Noun = Cat ** {...}
abstract Verb = Cat ** {...}
```
==Top-level grammar and lexicon==
The module ``Grammar`` collects all the category-specific modules into
a complete grammar:
```
abstract Grammar =
Adjective, Noun, Verb, ..., Structural, Idiom
```
The module ``Structural`` is a lexicon of structural words (function words),
such as determiners.
The module ``Idiom`` is a collection of idiomatic structures whose
implementation is very language-dependent. An example is existential
structures ("there is", "es gibt", "il y a", etc).
The module ``Lang`` combines ``Grammar`` with a ``Lexicon`` of
ca. 350 content words:
```
abstract Lang = Grammar, Lexicon
```
Using ``Lang`` instead of ``Grammar`` as a library may give
for free some words needed in an application. But its main purpose is to
help testing the resource library, rather than as a resource itself.
It does not even seem realistic to develop
a general-purpose multilingual resource lexicon.
The diagram in Figure 2 shows the structure of the API.
#BFIG
#GRAMMAR
#BCENTER
Figure 2. The resource syntax API.
#ECENTER
#EFIG
==Language-specific syntactic structures==
The API collected in ``Grammar`` has been designed to be implementable for
all languages in the resource package. It does contain some rules that
are strange or superfluous in some languages; for instance, the distinction
between definite and indefinite articles does not apply to Finnish and Russian.
But such rules are still easy to implement: they only create some superfluous
ambiguity in the languages in question.
But the library makes no claim that all languages should have exactly the same
abstract syntax. The common API is therefore extended by language-dependent
rules. The top level of each languages looks as follows (with English as example):
```
abstract English = Grammar, ExtraEngAbs, DictEngAbs
```
where ``ExtraEngAbs`` is a collection of syntactic structures specific to English,
and ``DictEngAbs`` is an English dictionary
(at the moment, it consists of ``IrregEngAbs``,
the irregular verbs of English). Each of these language-specific grammars has
the potential to grow into a full-scale grammar of the language. These grammars
can also be used as libraries, but the possibility of using functors is lost.
To give a better overview of language-specific structures,
modules like ``ExtraEngAbs``
are built from a language-independent module ``ExtraAbs``
by restricted inheritance:
```
abstract ExtraEngAbs = Extra [f,g,...]
```
Thus any category and function in ``Extra`` may be shared by a subset of all
languages. One can see this set-up as a matrix, which tells
what ``Extra`` structures
are implemented in what languages. For the common API in ``Grammar``, the matrix
is filled with 1's (everything is implemented in every language).
Language-specific extensions and the use of restricted
inheritance is a recent addition to the resource grammar library, and
has only been exploited in a very small scale so far.