Barton suggests:
> We could use strings for the C & D labels
> (c4) ["d2",d2];
> (d4) [- SIN(x), d2]
A string is a constant (like a number), and it would be a very bad idea
to have it evaluate to anything but itself. And it is in fact currently
an error to do so: "x":3 => Improper value assignment.
A slight variant of this would be to have to say labelval("d2"), but
this is far far too clumsy for something that should be very very
simple.