We could use strings for the C & D labels
(C1) inchar : "c";
(D1) c
(c1) outchar : "d";
(d1) d
(c2) diff(cos(x),x);
(d2) - SIN(x)
(c3) diff(%,x);
(d3) - COS(x)
(c4) ["d2",d2];
(d4) [- SIN(x), d2]
(c5) ["c3",c3];
(d5) [DIFF(%, x), c3]
(c6) diff(c2 * x,x);
(d6) c2
(c7) ev(%);
(d7) c2
(c8)
Visually, this is the same as "classic" maxima; if we went this route,
older documentation would only need to be fixed by
changing ci to "ci" and di to "di" when these symbols refer to labels.
BW