First of all, I found it a very interesting article. Common patterns, such as Visitor (double-dispatch) are means to get around the limitations of single-dispatch. It's nice to see multimethods take a step forward.
I have a couple of questions:
What are some representative languages which implemented multimethods?
Also, what algorithms are commonly used for matching a method to a set of parameters? Unlike in single-dispatch, there may be multiple methods which match to the same degree.