/// <summary>
/// 计算单独的表达式子
/// </summary>
/// <param name="formula"></param>
/// <returns></returns>
private double EvalExpression()
{
string formula = rule.Formula;
string expression = ReplaceFormulaDefinedVariable(formula);
ExpressionEval eval = new ExpressionEval(expression);
return(eval.EvaluateDouble());
}