code: private static void evaluateSubexpression(final Deque<String> operators, final Deque<String> values) {
          String operator = operators.pop();
          while (!(operator).equals(SeparatorToken.OPEN_PARENTHESIS.toString())) {
              values.push(getBooleanResultAsString(operator,
                      values.pop(),
                      values.pop()));
              operator = operators.pop();
          }
      }
docs: Evaluates a sub expression within a pair of parentheses and pushes its result onto the stack of values
  
  @param operators Stack of operators
  @param values    Stack of values