n a logical system;
more precisely, they are logical languages, in the sense that their
programs are sets of sentences in some logical system, and their
operational semantics is given by deduction in that logical
system. All recent OBJ languages use order sorted algebra, which also
provides a rigorous basis for user definable sub-types, exception
handling, multiple inheritance, overloading, multiple representations,
coercions, and more. They also support user definable mixfix syntax,
user definable executio