public void compileDerivative(
ExpressionProgram prog, int myIndex, ExpressionProgram deriv, Variable wrt) {
// The ExpressionCommand occurs in the program prog at the index indicated by myIndex.
// Add commands to deriv that will evaluate the derivative of this command with respect to
// the variable wrt. Note that the "Cmd" object is preceded in the program by commands that
// compute the lower and upper limits of the summation.
if (!sumExpr.dependsOn(wrt)) deriv.addConstant(0);
else {
int upper = prog.extent(myIndex - 1); // Size of expression giving the upper limit.
prog.copyExpression(myIndex - 1 - upper, deriv); // Copy lower limit exression to deriv.
prog.copyExpression(myIndex - 1, deriv); // Copy upper limit expression to deriv.
deriv.addCommandObject(new Cmd(sumVar, (ExpressionProgram) sumExpr.derivative(wrt)));
}
}
