module etox_module
implicit none

contains
  real function etox(x)
  implicit none
  real :: term
  real, intent (in) :: x
  integer :: nterm
  real, parameter :: tol = 1.0e-6

    etox = 1.0
    term = 1.0
    nterm = 0
    do
      nterm = nterm + 1
      term = (x/nterm)*term
      etox = etox + term
      if (abs(term)<=tol) exit
    end do
  end function etox
end module etox_module

program ch1306
use etox_module
implicit none
real, parameter :: x = 1.0
real :: y

  print *, ' Fortran intrinsic ', exp(x)
  y = etox(x)
  print *, ' User defined etox ', y
end program ch1306