Derives this expression with respect to the given variable.
Source
Expression derive(String toVar) {
MathFunction gDF;
Expression gD = g.derive(toVar);
if (!(gD is MathFunction)) {
// Build function again..
gDF = new CustomFunction('d${g.name}', g.args, gD);
} else {
gDF = (gD as MathFunction);
}
// Chain rule.
return new CompositeFunction(f, gDF) * f.derive(toVar);
}