[DNF2]

As far as I know, multiple dispatch is optional i clisp.

It also exists in R, but again, is optional (and slow), and is therefore mostly unused, which limits its usefulness, making it less used, etc. etc.

In Julia, it is default, so everything participates in multiple dispatch. It completely pervades the language, which is what makes it so useful.

[leephillips]

It is also available in Perl6, but there, as in CL, it is opt-in, and might slow things down.