  Distilling from a knowledge base only the part that is relevant to a subset
of alphabet, which is recognized as forgetting, has attracted extensive
interests in AI community. In standard propositional logic, a general algorithm
of forgetting and its computation-oriented investigation in various fragments
whose satisfiability are tractable are still lacking. The paper aims at filling
the gap. After exploring some basic properties of forgetting in propositional
logic, we present a resolution-based algorithm of forgetting for CNF fragment,
and some complexity results about forgetting in Horn, renamable Horn, q-Horn,
Krom, DNF and CNF fragments of propositional logic.
