We aim to execute fUML specifications written in Papyrus using the fUML reference implementation. This has not been challenging since an UML model containing a single Class diagram and one or more Activity Diagrams can be directly executed by calling the fUML constructor:
new Fuml(File);
However, it is difficult to specify complex behaviors without basic Integer operations. For example, 'equals', 'greater'...
I am aware that there is some library of such functions that can be selected using OpaqueBehaviors. Therefore, my question is: where and how can those such "basic operations" be found?
Sorry, this question was just pointed out to me recently. But, in case you haven't figured it out yet, I do have an answer for you.
The library you are looking for is the Foundational Model Library, which contains packages such as IntegerFunctions, BooleanFunctions, etc. The normative fUML specification includes a standard XMI file for this library (at http://www.omg.org/spec/FUML/20121019/fUML_Library.xmi, for fUML 1.1). However, to use the library in Papyrus, you need an Eclipse .uml version.
If you want, you can convert the normative XMI yourself: Download the file and open it in the Eclipse UML Editor (not Papyrus). Select File > Save As and save the file with a .uml extension (fUML_Library.uml). You should then be able to import this .uml file into Papyrus and reference the functions in it from your activity models. As long as you maintain the normative element IDs from the original XMI file, the reference implementation will know that these are standard library functions, for which it has built-in implementations.
However, there is actually a .uml version of this library already available for Papyrus. It is included with the additional Moka Model Execution component for Papyrus. If you install this component, then the fUML model library becomes available as a "registered library" that you can import into your model and use as above.
Note that Moka also includes an fUML execution engine that you can use to execute activity models directly within Papyrus (see https://wiki.eclipse.org/Papyrus/UserGuide/ModelExecution). However, any models created in Papyrus that can be executed with Moka can also be executed externally from Papyrus using the reference implementation, just as you have done before.