DownValues
 Usage
• DownValues[f]给出一个与所有为符号 f定义的downvalue相一致的变换规则列表.
Notes
• 你可以使用形式为DownValues[f] = list的赋值来指定downvalue. • 由 DownValues返回的列表的元素具有形式HoldPattern[lhs] :> rhs..
 Further Examples
This defines a downvalue for f.
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The definition is associated with f.
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 The definition is not associated with g; To associate the definition with g use TagSet (/:).
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