From: http://parasail-programming-language.blogspot.com/2009/10/using-in-parasail-to-implement-etc.html
>ParaSail skirts this issue by requiring the user to define only one operator, >"=?", which returns a value from the enumeration Less, Equal, Greater, >Unordered. The comparison and equality operators are defined in terms of "=?" >[...] "Unordered" is used for types with only a partial ordering (or no >ordering at all). "Ordered" is defined as the subtype containing only the >values "Equal," "Less," and "Greater." The typical generic "Sort" operation >would expect a "=?" operator whose result is guaranteed (via a postcondition) >to be within the "Ordered" subtype. The "=?" operator can be used explicitly >if desired, which is often convenient when searching a binary tree:< Bye, bearophile
