Equivalent literal propagation in the DLL procedure

We propose a simple data structure to represent all equivalent literals such as l1 $ l2 in a CNF formula F , and implement a special look-ahead technique, called equivalency reasoning, to propagate these equivalent literals in F in order to get other equivalent literals and to simplify F . Equivalent literal propagation remedies the ineffectiveness of unit propagation on equivalent literals and makes easy many SAT problems containing both usual CNF clauses and the so-called equivalency clauses (Ex-OR or modulo 2 arithmetics). Our approach is also compared with general CSP look-back techniques on these problems.