对选项，参数和变量来说，最有趣的变化是移除了值属性；现在值是一个Xpath表达式的计算结果，该表达式由一个选择属性定义。
The most interesting change with options, parameters, and variables is the removal of the value attribute; values are now the result of evaluating an XPath expression defined by a select attribute.
第二个过滤器负责移除任何表达式左侧为一个基本类型或null型的列表条目。
The second filter removes any list items where the left side of the expression is a primitive type or a null.
当希望移除对在约束表达式所包含的列或列组合中输入的值的唯一性要求时，可删除唯一约束。
Delete a unique constraint when you want to remove the requirement for uniqueness for values entered in the column or combination of columns included in the constraint expression.
此优化将不变量子表达式从循环体中移除。
This optimization removes invariant subexpressions from the body of a loop.
因为其表意不明，这个表达式在Python3.0中被移除；
Due to its obscurity, this expression is removed in Python 3.0;
因为其表意不明，这个表达式在Python3.0中被移除；
Due to its obscurity, this expression is removed in Python 3.0;
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