Apparently, Alexander Stepanov has stated the following in an interview:
“I find OOP technically unsound. It attempts to decompose the world in terms of interfaces that vary on a single type. To deal with the real problems you need multisorted algebras - families of interfaces that span multiple types.” [Emphasis added.]
Ignoring his statement regarding OOP for a moment, what are "multisorted algebras", beyond his terse definition, and can you give a practical example of how they are used (in the language of your choice)?