Spu_promote converts a value to a vector, but only defines one element.
spu_promote将一个值转换成向量，不过只定义了一个元素。
spu_extract pulls out a specific element from a vector and returns it as a scalar.
spu_extract 会从向量中取出某个指定的元素，并将其作为一个标量返回。
Spu_insert is used to put a scalar value into a specific element of a vector.
spu_insert用来将一个标量值放入向量的某个特定元素中。
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