INSTANCE: Set U of variables, collection C of conjunctive
clauses of at most 3 literals, where a literal is a variable or a
negated variable in U.
SOLUTION: A renamable Horn subformula C' of C, i.e., a
subset
such that there exists a subset
of the variables such that switching the literals in
makes C' a Horn Boolean formula, where
.
MEASURE: The cardinality of the subformula, i.e., .