In this paper,a simple expression E,i.e.,a term or an atom,is viewed as a set,the"so-called"generated set of E,of the Herbrand universe U.or Herbrand base BLof the language L.The authors give a criteria to determine whether an expression coversanother one,and present the notion of decomposition expressions and the decomposition algorithm on which a necessary and sufficient condition for the covering relation between one and several other expressions is based.Then,they show how to partition a union of the sets generated by the simple expressions according to the partition formula in the set theory.Finally,they generalize generis these above results into the conjunctions through defining a transformation which converts a conjunction into a simple expression.This paper provides the theoretic foundation of the procedural semantics of authors'annotated logic program.