However, RMS has a good point that the subrs really just wrap up C
functions, so it's not clear that this makes sense, and things should
not be less efficient or convenient when the argument is not needed.
I wonder if there's some advantage to making a second kind of subr that
holds an extra argument and prepends it before calling the C function.
Of course, this adds no new expressive power, because it's basically
just a lambda expression.
It's a good question what sort of support for this would be desirable in
compilers and interpreters. Perhaps the lambda expression would be
implemented efficiently enough that there's no performance advantage.
Even if that's true, it may be helpful to humans to do something special
so that they look like primitives rather than some lambda expression.