Experimental`ValueFunction
 Usage
• ValueFunction[symb] represents a function to be applied whenever the symbol symb gets a new value.
Notes
• The assignment ValueFunction[symb] = f specifies that whenever symb gets a new value val, the expression f[symb, val] should be evaluated. • If the value of symb is cleared, f[symb] is evaluated. • ValueFunction takes account of all ways that the value of a symbol can be changed, not just Set.
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