## About
This is an *improved* re-implementation of the combined approach for CQ answering over RSA ontologies described in [[1](#references)].
> Please note that the prototype mentioned in [[1](#references)] is not available (and the contributors to this repository have never seen it);
> therefore, this "re-implementation" could be completely different from that prototype (potentially using different tools and programming language).
## Preliminaries
In order to use this program you need to have [RDFox](https://www.oxfordsemantic.tech/product) available in your system, along with *a valid license*.
RDFox is proprietary software and as such we are not able to distribute it along with our code.
Please refer to [this link](https://www.oxfordsemantic.tech/tryrdfoxforfree) to request a free trial.
This software has been developed and tested with RDFox v5.2.1
## Changes introduced
We tried to implement the system as close as possible to the theoretical description provided in [[1](#references)].
Regardless, we had to deal with the fact that we are using different tools to carry out reasoning tasks and we are probably using a different language to implement the system.
The following is a (non exhaustive) summary of fixes (🔧), changes (🔄) and improvements (⚡), we introduced along the way:
+ 🔄 [RDFox](https://www.oxfordsemantic.tech/product) is used instead of DLV as the underlying LP engine.
+ âš¡ The system accepts unrestricted OWL ontologies as input and takes care of normalising and approximating the ontology to RSA.
At the time of writing, two approximation algorithms are provided, to compute a sound (or complete) set of answer to the input queries, respectively.
+ âš¡ The different steps of the combined approach (namely, the canonical model computation and the filtering step) are executed in isolation using different *named graphs*.
This allows us to reuse partial products of the computation and can even be used to parellalise filtering and answering steps.
+ 🔧 In Def.4, the definition of built-in predicate `notIn` is wrong and should reflect the implicit semantics implied by the name, i.e.,
> let [...] `notIn` be a built-in predicate which holds when the first argument is **not** an element of the set given as second argument
This has been fixed by (1) introducing a built-in predicate `In` (note that instances of `In` can be computed beforehand since they only depend on the input ontology), and (2) implement `notIn` as the negation of `In` using RDFox *NaF* built-in support.
+ 🔄 Top (`owl:Thing`) axiomatisation is performed introducing rules as follows.
Given `p` predicate (arity *n*) *in the original ontology*, the following rule is introduced:
```
owl:Thing[?X1], ..., owl:Thing[?Xn] :- p(?X1, ..., ?Xn) .
```
Note that, by definition, arity can be either 1 or 2.
+ 🔄 Equality axiomatisation is performed introducing the following rules:
```
rsacomb:congruent[?X, ?X] :- owl:Thing[?X] .
rsacomb:congruent[?Y, ?X] :- rsacomb:congruent[?X, ?Y] .
rsacomb:congruent[?X, ?Z] :- rsacomb:congruent[?X, ?Y], rsacomb:congruent[?Y, ?Z] .
```
defining equivalence as a congruence relation over terms in the ontology.
Substitution rules propagate the equivalence to all existing atoms.
+ 🔧 In Def. 4, the definition of built-in predicate `NI` is not consistent with its use in Table 3 and related description in Sec. 4.2.
We redefined `NI` as the set of all constants that are *equal* to a constant in the original ontology (according to the internal equality predicate `rsa:congruent`).
Note that, in this scenario, there is no need to introduce `NI` instances as facts in the system;
instead we can add a rule to populate the new predicate:
```
rsa:NI[?X] :- rsa:congruent[?X, ?Y], rsa:named[?Y] .
```
where `rsa:named` is an internal predicate keeping track of all constants in the original ontology.
+ âš¡ In Def. 3, regarding the generation of the logic program used for the RSA check, only T5 axioms involving an unsafe role will introduce the internal predicates `PE` and `U`.
+ âš¡ Both in the canonical model and the filtering program computations, rules without a body are loaded into RDFox as facts.
+ ⚡ The `cycle` function introduced in Def.4 establishing the direction of the *unraveling* of loops is defined over triples `(A,R,B)`. We are currently limiting the triple only to those appearing in a T5 axiom `A ⊑ ∃R.B`. Note that this greatly limits the size of cycle for a given triple, and as a consequence limits the number of rules used to compute the canonical model.
## Using the software
We assume you followed [these steps](https://docs.oxfordsemantic.tech/getting-started.html#getting-started) in order to setup RDFox on your personal machine and in particular you know the path to the `JRDFox.jar` library that comes with the distribution.
Alternatively, run the following commands from the root of the project to install RDFox locally.
Download links for specific versions and operating systems can be found [here](https://www.oxfordsemantic.tech/downloads).
```{.bash}
mkdir -p lib && pushd lib
wget https://rdfox-distribution.s3.eu-west-2.amazonaws.com/release/v5.2.1/RDFox-linux-x86_64-5.2.1.zip
unzip RDFox-linux-x86_64-5.2.1.zip
ln -s RDFox-linux-x86_64-5.2.1.zip/lib/JRDFox.jar
popd
```
### Provide RDFox license
The [documentation](https://docs.oxfordsemantic.tech/features-and-requirements.html#license-key), describes several ways to provide the license to RDFox.
One easy way is to put your license key in a file `RDFox.lic` in `$HOME/.RDFox/`, with adequate read permissions for the user executing the program.
### Compiling and running the project
The project uses [sbt](https://www.scala-sbt.org/) to manage dependences.
To compile the project run the following from the base directory:
```
sbt compile
```
The project uses the sbt plugin [sbt-assembly](https://github.com/sbt/sbt-assembly) to produce a fat jar with all the required dependences.
Run the following from the base directory of the project to produce a standalone `jar` file.
```
sbt assembly
```
The output of the command will print the location of the produced jar.
Note that the fat jar file distributed with this repository excludes the RDFox as a dependency. Provided that you have the RDFox setup on your machine, you can run the program as follows
```
java -cp : uk.ac.ox.cs.rsacomb.RSAComb [
