text: #ifndef STAN_MATH_REV_CORE_OPERATOR_NOT_EQUAL_HPP
  #define STAN_MATH_REV_CORE_OPERATOR_NOT_EQUAL_HPP
  
  #include <stan/math/rev/core/var.hpp>
  
  namespace stan {
    namespace math {
  
      /**
       * Inequality operator comparing two variables' values (C++).
       *
         \f[
         \mbox{operator!=}(x, y) =
         \begin{cases}
           0 & \mbox{if } x = y\\
           1 & \mbox{if } x \neq y \\[6pt]
           0 & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
         \end{cases}
         \f]
       *
       * @param a First variable.
       * @param b Second variable.
       * @return True if the first variable's value is not the same as the
       * second's.
       */
      inline bool operator!=(const var& a, const var& b) {
        return a.val() != b.val();
      }
  
      /**
       * Inequality operator comparing a variable's value and a double
       * (C++).
       *
       * @param a First variable.
       * @param b Second value.
       * @return True if the first variable's value is not the same as the
       * second value.
       */
      inline bool operator!=(const var& a, double b) {
        return a.val() != b;
      }
  
      /**
       * Inequality operator comparing a double and a variable's value
       * (C++).
       *
       * @param a First value.
       * @param b Second variable.
       * @return True if the first value is not the same as the
       * second variable's value.
       */
      inline bool operator!=(double a, const var& b) {
        return a != b.val();
      }
  
    }
  }
  #endif
  