> > But when designing a general utility, for all kinds of programs to use,
> > it is inappropriate to say "storing null elements is not useful".
>
> I'm afraid we'll have to disagree here.

In some of my uses of the gl_list module, the element is in fact a small
integer, cast to an 'uintptr_t' and then further cast to a 'const void *'.
If the API would forbid storing a NULL pointer, my specialized container
type would need a workaround for storing the integer value 0.