c+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
c-----------------------<<<    secant.f       >>>-----------------------
c-----------------------------------------------------------------------
C                        I     I      I    I
      subroutine secant(x_old,x_new,y_old,y_new)
C                         _____J       ____J
C                        |       O    |    |  
C                        |       |    |    |  

      implicit none

      double precision temp
      double precision x_old,x_new
      double precision y_old,y_new

       temp = x_new
      x_new = x_new - y_new*(x_new-x_old)/(y_new-y_old)

      y_old = y_new
      x_old = temp

      end
c+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
