docs: public static int[] getIndexesOfFreeVariable(Expression2 expression, String variableName) {
      List<Integer> indexes = Lists.newArrayList();
      ScopeSet scopes = getScopes(expression);
      for (int i = 0; i < expression.size(); i++) {
        Scope scope = scopes.getScope(i);
        String varName = getFreeVariable(expression, i, scope);
        if (varName != null && varName.equals(variableName)) {
          indexes.add(i);
        }
      }
      return Ints.toArray(indexes);
    }