text: #ifndef STAN_MATH_FWD_SCAL_FUN_LOG_DIFF_EXP_HPP
  #define STAN_MATH_FWD_SCAL_FUN_LOG_DIFF_EXP_HPP
  
  #include <stan/math/fwd/core.hpp>
  #include <stan/math/prim/scal/fun/constants.hpp>
  #include <stan/math/prim/scal/fun/expm1.hpp>
  #include <stan/math/prim/scal/fun/log_diff_exp.hpp>
  
  namespace stan {
    namespace math {
  
      template <typename T> inline fvar<T>
      log_diff_exp(const fvar<T>& x1, const fvar<T>& x2) {
         using std::exp;
         if (x1.val_ <= x2.val_)
           return fvar<T>(NOT_A_NUMBER, NOT_A_NUMBER);
         return fvar<T>(log_diff_exp(x1.val_, x2.val_),
                        -(x1.d_ / expm1(x2.val_ - x1.val_)
                          + x2.d_ / expm1(x1.val_ - x2.val_)));
      }
  
      template <typename T1, typename T2> inline fvar<T2>
      log_diff_exp(const T1& x1, const fvar<T2>& x2) {
        using std::exp;
        if (x1 <= x2.val_)
          return fvar<T2>(NOT_A_NUMBER, NOT_A_NUMBER);
        return fvar<T2>(log_diff_exp(x1, x2.val_),
                        -x2.d_ / expm1(x1 - x2.val_));
      }
  
      template <typename T1, typename T2> inline fvar<T1>
      log_diff_exp(const fvar<T1>& x1, const T2& x2) {
        using std::exp;
        if (x1.val_ <= x2)
          return fvar<T1>(NOT_A_NUMBER, NOT_A_NUMBER);
        return fvar<T1>(log_diff_exp(x1.val_, x2),
                        -x1.d_ / expm1(x2 - x1.val_));
      }
    }
  }
  #endif
  