Erratum for an August 2007 Fortran Forum article

The article "A Fortran binding for the GNU scientific library" in the
August 2007 issue of SIGPLAN Fortran Forum contains an error in the
section "Non-interoperable function arguments and Fortran polymorphism".
Under the heading "Usage: contrasting Fortran and C", a function my_fun
is described (in the second table of the subsection). This function must
be amended to read 

function my_fun(x,params)
   real(fgsl_double) :: my_fun
   real(fgsl_double), intent(in) :: x
   class(fgsl_void) :: params
   select type (params)
   class is (my_void)
      my_fun = params%d(1) + &
            x*(params%d(2) + ...))))
   class default
      stop 'incorrect extension of fgsl_void'
   end select
end function my fun

The reason for this lies in the fact that the library internally uses a
polymorphic pointer of base class to the function parameters. According
to section 12.4.1.2 of the Fortran 2003 standard, the actual argument
used internally must be of the same type as the dummy argument, or one
of its extensions.
As a consequence, the dummy argument used in the user-provided function
must also be of base class. Making use of select type is then unavoidable.
This particular example is not a case of run time type identification
(RTTI), but of run time class identification (perhaps to be abbreviated
as RTCI).