Specializing on Compile Time Constants

[dsimcha]

I'm working on a mathematical expression interpreter for D, which would allow
for closed form mathematical expressions to be specified as string literals at
runtime and be evaluated.  For example:

MathExp myExpression = mathExp("x^2 + e^cos(-x) - 2 * sqrt(pi)", "x");
writeln(myExpression(2));  // Does exactly what you think it does.

I've found the syntax so convenient that I'd like to transparently specialize
it on strings known at compile time.  The idea is that, when the expression is
hard-coded, you will still be able to use the MathExp interface, but your
expression will evaluate at the full speed of a statically compiled function.
 Is there any way to test whether the value of an argument to a template
function is known at compile time and specialize on this?