The 1998 POPL paper:

@INPROCEEDINGS{flatt:mixin,
AUTHOR = " M. Flatt and S. Krishnamurthi and M. Felleisen",
TITLE = "Classes and Mixins",
BOOKTITLE = "POPL '98",
YEAR = "1998",
ADDRESS = "",
PAGES = "",
EDITOR = "",
PUBLISHER = "ACM Press"
}
http://www.ccs.neu.edu/research/demeter/related-work/rice

talks about uniform extension of a group of classes. Introduces
MixedJava, as an extension of Java with mixins.
In MixedJava, super dispatches are dynamic, since the super mixin
for a super expression is not statically allow.
Therefore, mixins can be composed dynamically like adjusters
and the new visitors.

The Rice team also works on the visitor design pattern.

-- Karl