code: private static boolean containsComplement(final LinkedHashSet<Formula> formulas, final Formula f) {
      return formulas.contains(f.negate());
    }
docs: Returns {@code true} if a given list of formulas contains the negation of a  given formula,
  {@code false} otherwise.
  @param formulas the list of formulas
  @param f        the formula
  @return {@code true} if a given list of formulas contains a given formula, {@code false} otherwise