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