Extend the feature modeling language Cross tree dependencies

[johmara]

Describe the need of your request

In order to implement a complete feature model in CLAFER we need to be able to define cross tree constraints, optionality? and OR/XOR relationships. On Clafer.org this is described.

rootfeature
    A ?
        or A1
           A2
    B ?
        [A]
    C
        xor C1
            C2
[B=>A]

explanation of cross-tree constraints in feature models:
https://link.springer.com/chapter/10.1007/978-3-031-23669-3_11

Proposed solution

Extend the syntax (concrete and abstract) so that cross-tree constraints, feature optionality ("?") and OR/XOR grouping are supported in the feature model editor.

Whenever an operator is added to a feature, the system should differentiate and highlight the operator correctly. Additionally, features that are connected with an OR or XOR operator should be visually grouped.

To get started, several places of the code needs to be edited. First, the grammar needs to be edited to allow additional operators. The grammar of the feature model can be found in src\main\java\se\isselab\HAnS\featureModel\FeatureModel.bnf. Second, the lexer needs to be configured to be able to identify the tokens and transition between the different states in src\main\java\se\isselab\HAnS\featureModel\FeatureModel.flex. The Grammar-Kit Plugin is required to receive syntax highlighting support and acquire tools to generate the lexer files read by the plugin during execution. For that, simply right-click on the .bnf and .flex file and run Jflex to generate the parser and lexer respectively. Finally, the newly defined tokens need to additionally be defined in the interface FeatureModelTypes. If done correctly, the plugin should recognize the new grammar when editing the feature model using the plugin.
Final edits must be then made to change the syntax highlighting and/or the formatting of the file.

Note: When generating the lexer file, you may run into several errors stating, that an import is missing and two variables are not defined in the generated file. This may happen due to an outdated JFlex version used by the Grammar-kit plugin. As a fix, you can manually insert the missing import and delete the 2 variables throwing the error.

[thorstenberger]

Add Boolean constraint support:
http://t3-necsis.cs.uwaterloo.ca:8091/Clafer%20Cheat%20Sheet
http://t3-necsis.cs.uwaterloo.ca:8091/Clafer%20Cheat%20Sheet#expressions