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ValueFunction
ValueFunction[symb]
represents a function to be applied whenever the symbol symb gets a new value.
更多信息
- To use
, you first need to load the Experimental Functions Package using Needs["Experimental`"]. - The assignment
specifies that whenever symb gets a new value val, the expression 
should be evaluated. - If the value of symb is cleared,
is evaluated. 
takes account of all ways that the value of a symbol can be changed, not just Set.
