code: protected Set<MavenProject> getExecutedModules(MavenProject rootModule) {
          String executionKey = createExecutionKey( execution );
          String executedModulesContextKey = AbstractProjectMojo.class.getName() + "#executedModules";
          Map<String, Set<MavenProject>> executedProjectsPerExecutionKey =
                  (Map<String, Set<MavenProject>>) rootModule.getContextValue(executedModulesContextKey);
          if (executedProjectsPerExecutionKey == null) {
              executedProjectsPerExecutionKey = new HashMap<>();
              rootModule.setContextValue(executedModulesContextKey, executedProjectsPerExecutionKey);
          }
          Set<MavenProject> executedProjects = executedProjectsPerExecutionKey.get(executionKey);
          if (executedProjects == null) {
              executedProjects = new HashSet<>();
              executedProjectsPerExecutionKey.put(executionKey, executedProjects);
          }
          return executedProjects;
      }
docs: Determine the already executed modules for a given root module.
  
  @param rootModule The root module.
  @return The set of already executed modules belonging to the root module.