An operational methodolgy, founded on {predicate calcuLUs}, dealing with descripitive expressions that are equivalent to the operations of relational algebra. {Codd' s reduction algorithm} can convert from {relational calcuLUs} to relational algebra. Two forms of the relational calcuLUs exist: the {tuple calcuLUs} and the {domain calcuLUs}. ["An Introduction To Database Systems" (6th ed), C. J. Date, Addison Wesley]. (1998-10-05)