// and this could lead to cycles where a thread waits for
            // a task to finish, then execute another one which waits
            // for the initial task...
            // In order to work around that problem, we override the
            // invokeAll method, so that whenever the method is called,
            // the pool core size will be incremented before submitting

                }

                private BuilderProblem toProblem(ModelProblem problem) {
                    return new BuilderProblem() {
                        @Override
                        public String getSource() {
                            return problem.getSource();
                        }

                        @Override