text: #ifndef STAN_LANG_AST_NODE_LUB_IDX_HPP
  #define STAN_LANG_AST_NODE_LUB_IDX_HPP
  
  #include "expression.hpp"
  
  namespace stan {
  namespace lang {
  
  /**
   * AST structure for lower and upper bounds.
   */
  struct lub_idx {
    /**
     * Lower bound.
     */
    expression lb_;
  
    /**
     * Upper bound.
     */
    expression ub_;
  
    /**
     * Construct a default (nil valued) lower and upper bound index.
     */
    lub_idx();
  
    /**
     * Construt a lower and upper bound index with the specified
     * bounds.
     *
     * @param lb lower bound
     * @param ub upper bound
     */
    lub_idx(const expression& lb, const expression& ub);
  };
  
  }  // namespace lang
  }  // namespace stan
  #endif
  