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based on an earlier version of the Wolfram Language.
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represents a function to be applied whenever the symbol symb gets a new value.
  • 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.