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一个例外是假如对象包含函数，那么肯定不会被序列化并保存。
One exception is that if your object has functions, those can obviously not be serialized and saved.
注意精确性与例外情况的处理在 MASS 函数与系统库函数中可能是不一样的。
Note that the accuracy and exception handling might not be identical in MASS functions and system library functions.
一个例外是ST_Distance函数，它必须出现在针对一个任意距离的小于比较式中。
An exception is the function ST_Distance, which must be used in a less-than comparison with an arbitrary distance.
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