docs: add explantions about the aggreate functions (#59)

Author: MiriamGaus

README.md

@@ -1,4 +1,5 @@
 # UVL - Universal Variability Language
+
 This is a small default library used to parse and print the Universal Variability Language (UVL).
 
 Under the hood it uses [ANTLR4](https://www.antlr.org/) as the parsing library.
@@ -9,16 +10,21 @@ The grammar in EBNF form is located in `uvl/UVL.g4` and the modifications for Ja
 On a high level, each feature model in UVL consists of five optional separated elements:
 
 1. **A list of used language levels**
-The model can use different concepts which are part of language levels. These levels can either be enumerated with the `include` keyword or be implicit.
+   The model can use different concepts which are part of language levels. These levels can either be enumerated with the `include` keyword or be implicit.
 2. **A namespace which can be used for references in other models**
 3. **A list of imports that can be used to reference external feature models**
-The models are referenced by their file name and can be given an alias using a Java import like syntax.
-External models in subdirectories can be referenced like this: subdir.filename as fn
+   The models are referenced by their file name and can be given an alias using a Java import like syntax.
+   External models in subdirectories can be referenced like this: subdir.filename as fn
 4. **The tree hierarchy consisting of: features, group types, and attributes whose relations are specified using nesting (indentation)**
-Groups may have an arbitrary number of features as child nodes. A feature can also have a feature cardinality.
-Attributes consist of a key-value pair whose key is always a string and its value may be a boolean, number, string, a list attributes, a vector, or a constraint. If the value is a constraint the key must be `constraint`. If the value is a list of constraints the key must be `constraints`
+   Groups may have an arbitrary number of features as child nodes. A feature can also have a feature cardinality.
+   Attributes consist of a key-value pair whose key is always a string and its value may be a boolean, number, string, a list attributes, a vector, or a constraint. If the value is a constraint the key must be `constraint`. If the value is a list of constraints the key must be `constraints`
 5. **Cross-tree constraints**
-Cross-tree constraints may be arbitrary propositional formulas with the following symbols: => (implies), <=> (iff), & (and), | (or), ! (not), or brackets. Through the usage of language levels cross-tree constraints can also contain equations (<,>,==) which consist of expressions (+,-,*,/) with numbers or numerical feature attributes as literals and aggregate functions (avg, sum).
+   Cross-tree constraints may be arbitrary propositional formulas with the following symbols: => (implies), <=> (iff), & (and), | (or), ! (not), or brackets. Through the usage of language levels cross-tree constraints can also contain equations (<,>,==) which consist of expressions (+,-,\*,/) with numbers or numerical feature attributes as literals and aggregate functions (avg, sum).
+   The sum and avg functions compute the total or average of a numeric attribute across all instances, optionally limited to a specified feature subtree, using the syntax e.g.
+   ```
+    constraints
+      sum(att, rootFeature).
+   ```
 
 The following snippet shows a simplified server architecture in UVL. We provide more examples (e.g., to show the composition mechanism) in [https://github.com/Universal-Variability-Language/uvl-models/tree/main/Feature_Models](https://github.com/Universal-Variability-Language/uvl-models/tree/main/Feature_Models).
 
@@ -50,28 +56,31 @@ constraints
 ```
 
 In this snippet, we can recognize the following elements:
-* The feature `Server` is abstract (i.e., corresponds to no implementation artifact.
-* Each `Server` requires a `FileSystem`and an `OperatingSystem` denoted by the *mandatory* group
-* The `Server` may have `Logging` denoted by the *optional* group
-* A `FileSystem` requires at least one type of `NTFS`, `APFS`, and `Ext4` denoted by the *or* group
-* An `OperatingSystem` has exactly one type of `Windows`, `macOS`, and `Debian`denoted by the *alternative* group
-* `Logging` has the feature attribute `log_level` attached which is set to "warn"
-* `Windows` requires `NTFS` denoted by the first cross-tree constraint
-* `macOS`requires `APFS`
+
+- The feature `Server` is abstract (i.e., corresponds to no implementation artifact.
+- Each `Server` requires a `FileSystem`and an `OperatingSystem` denoted by the _mandatory_ group
+- The `Server` may have `Logging` denoted by the _optional_ group
+- A `FileSystem` requires at least one type of `NTFS`, `APFS`, and `Ext4` denoted by the _or_ group
+- An `OperatingSystem` has exactly one type of `Windows`, `macOS`, and `Debian`denoted by the _alternative_ group
+- `Logging` has the feature attribute `log_level` attached which is set to "warn"
+- `Windows` requires `NTFS` denoted by the first cross-tree constraint
+- `macOS`requires `APFS`
 
 ## Building a jar
 
 The library is a maven project and can therefore be build with maven. To update the generated parser classes and create a jar with all necessary dependencies, use:
+
 ```
 mvn clean compile assembly:single
 ```
 
 The `target/uvl-parser-1.0-SNAPSHOT-jar-with-dependencies.jar` includes all dependencies.
 
 ## Usage from Java
+
 The class `de.vill.main.UVLModelFactory` exposes the static method `parse(String)` which will return an instance of a `de.vill.model.FeatureModel` class. If there is something wrong, a `de.vill.exception.ParseError` is thrown. The parser tries to parse the whole model, even if there are errors. If there are multiple errors, a `de.vill.exception.ParseErrorList` is returned which contains all errors that occurred.
 A model can be printed with the `toString()` method of the `de.vill.model.FeatureModel` object.
-The following snippet shows a minimal example to read and write UVL models using the jar. More usage examples that also show how to use the acquired UVLModel object can be found in [src/main/java/de/vill/main/Example.java](https://github.com/Universal-Variability-Language/uvl-parser2.0/blob/main/src/main/java/de/vill/main/Example.java)
+The following snippet shows a minimal example to read and write UVL models using the jar. More usage examples that also show how to use the acquired UVLModel object can be found in [src/main/java/de/vill/main/Example.java](https://github.com/Universal-Variability-Language/java-fm-metamodel/blob/main/src/main/java/de/vill/main/Example.java)
 
 ```Java
 // Read
@@ -85,17 +94,19 @@ FeatureModel featureModel = uvlModelFactory.parse(content);
 String uvlModel = featureModel.toString();
 Path filePath = Paths.get(featureModel.getNamespace() + ".uvl");
 Files.write(filePath, uvlModel.getBytes());
-``` 
+```
 
 ## Links
+
 UVL models:
-* https://github.com/Universal-Variability-Language/uvl-models
+
+- https://github.com/Universal-Variability-Language/uvl-models
 
 Other parsers:
-* https://github.com/Universal-Variability-Language/uvl-parser *deprecated, Initial UVL Parser, based on Clojure and instaparse* **UVL-Parser**
-* https://github.com/diverso-lab/uvl-diverso/ *Under development, Antlr4 Parser* **Diverso Lab**
 
-Usage of UVL:
-* https://github.com/FeatureIDE/FeatureIDE *Feature modelling tool* 
+- https://github.com/Universal-Variability-Language/uvl-parser _deprecated, Initial UVL Parser, based on Clojure and instaparse_ **UVL-Parser**
+- https://github.com/diverso-lab/uvl-diverso/ _Under development, Antlr4 Parser_ **Diverso Lab**
 
+Usage of UVL:
 
+- https://github.com/FeatureIDE/FeatureIDE _Feature modelling tool_