MathExp: KISS or All-Out?

[dsimcha]

I'm working on some mathy modules that I'd like to eventually contribute to
Phobos, or, if they're too niche, to a standalone lib.  One that I've alluded
to here in the past few days is MathExp.  Basically what it does is
parse/interpret/evaluate/manipulate mathematical expressions at runtime.
Here's a summary of what I've got working so far:

MathExp numerator = mathExp("x^2 * cos(x)", "x");
MathExp denominator = mathExp("exp(-x)", "x");
MathExp ratio = numerator / denominator;

// Print the result of
// (x^2 * cos(x)) / (exp(-x)) evaluated at x = 5.
writeln(ratio(5));

// Compute a derivative symbolically.
MathExp derivDenominator = denominator.derivative("x");

I'm trying to figure out which one makes more sense in the grand scheme of
things.  MathExp might be best kept very simple and stupid and pretty much
left as is, possibly even with the symbolic differentiation capabilities
removed (though these contribute surprisingly little to the weight of the
code; I wrote most of them in one evening just to see if I could).  It would
be easy to use, understand and debug, but be very stupid and have virtually no
semantic understanding of the expressions it's manipulating.  It wouldn't even
be able to simplify, for example, (1 + x + 1) -> (x + 2).

On the other hand, I could take this to the nth degree and add things like
decent printing capabilities (right now, my toString() method is a huge
kludge, is intended for debugging purposes, and puts in tons of unnecessary
parentheses).  I could figure out how to add some semantic analysis to
simplify expressions, maybe symbolic equation solving in some simple cases,
etc.  The downside to this is that I would be reinventing the computer algebra
system in D, which might be a bit too much for a small hobby project that's
intended as a Phobos module.  I'd likely make the code an order of magnitude
more complicated than it is now and might end up reinventing a square wheel or
writing some really buggy code.

If the goal here is to build a Phobos module or standalone plain old library,
do you think the KISS approach or the full-fledged approach is more useful to
the D community?