Changing the base of logs in Maxima

From http://math-blog.com/2007/06/04/a-10-minute-tutorial-for-solving-math-problems-with-maxima/,  you can change the base of logs by using the formula below. Log in Maxima is to the base of e. Changing to the log of base 10 is as follows -

log10(x):= log(x)/log(10);

So for logs to the base 9, below is what you should be using for the software

log9(x):= log(x)/log(9.0);

For example,

log9(x):= log(x)/log(9.0);
log9(81.0);

which should give an answer of 2.0. I had to use a decimal point for most of the  numbers in the brackets.