docs: public void getAllExpressions(Collection<Expression2> accumulator) {
      accumulator.add(rootExpression);
  
      for (int i = 0; i < substitutions.size(); i++) {
        substitutions.get(i).getAllExpressions(accumulator);
      }
  
      for (int i = 0; i < lefts.size(); i++) {
        lefts.get(i).getAllExpressions(accumulator);
        rights.get(i).getAllExpressions(accumulator);
      }
    }