Bug: `rules.xy` access within the objective generates non-compilable code

[maxkratz]

Loosely related to #220.

See the newly created test case: https://github.com/Echtzeitsysteme/gips-tests/blob/eb501311d486dd079da6357ef11f8fcbbf9ade55/gipsl.all.build.objective.keywordaccess/src/gipsl/all/build/objective/keywordaccess/Model.gipsl