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program optional_example
implicit none
real(8) :: result
call power_function(3.0_8, result)
if (result /= 9.0_8) error stop
call power_function(3.0_8, result, 3)
if (result /= 27.0_8) error stop
call interface_trouble(5)
contains
subroutine power_function(base, res, exponent)
implicit none
real(8), intent(in) :: base
integer, intent(in), optional :: exponent
real(8), intent(out) :: res
integer :: exp_value
if (present(exponent)) then
exp_value = exponent
else
exp_value = 2
end if
res = base ** exp_value
end subroutine power_function
recursive subroutine interface_trouble(n,t)
implicit none
integer, intent(in) :: n
real(8), intent(out), optional :: t
if(n <= 0) return
call interface_trouble(n-1, t)
end subroutine interface_trouble
end program optional_example
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