Polymorphism is another very important and related notion.
It is clear that order sorted algebra is very suitable for subtype polymorphism. It also provides a solution to the other kinds of polymorphism. But some problems are still open, e.g. the relation between parametric polymorphism and parameterized algebraic specifications. Technical report, SRI International, Computer Science Lab, 1990. Instead of using formulae, these sets can be defined by means of automata or grammars.