code: public boolean containsFormula(final Formula formula) {
      for (final Formula f : this.formulas)
        if (formula.equals(f))
          return true;
      return false;
    }
docs: Returns {@code true} if this formula list contains a given formula, {@code false} otherwise.  Formulas are only
  checked on the top-level, e.g. a sub-formula check is not performed.
  @param formula the formula
  @return {@code true} if this formula list contains a given formula