code: public Formula formula(final FormulaFactory f) {
      if (this.operator != FType.AND && this.operator != FType.OR)
        throw new IllegalStateException("Illegal operator for formula list formula construction: " + this.operator);
      if (this.formula == null) {
        if (this.operator == FType.AND)
          this.formula = f.and(this.formulas);
        else
          this.formula = f.or(this.formulas);
      }
      return this.formula;
    }
docs: Returns this formula list as a formula if there is an operator.
  @param f the formula factory
  @return this formula list as a formula
  @throws IllegalStateException if the operator is not n-ary