In addition to functional and system theories, Full Maude also supports object-oriented theories. Their structure is the same as that of object-oriented modules. Object-oriented theories can have classes, subclass relationships, and messages. These object-oriented notions may be useful for the definition of theories; for example, the following theory CELL specifies the theory of classes with at least one attribute of any sort.
(oth CELL is
sort Elt .
class Cell | contents : Elt .
endoth)