BUILT-IN MATHEMATICA SYMBOL
$Pre
$Pre
is a global variable whose value, if set, is applied to every input expression.
- Unless $Pre is assigned to be a function which holds its arguments unevaluated, input expressions will be evaluated before $Pre is applied, so the effect of $Pre will be the same as $Post.
- $Pre is applied to expressions, while $PreRead is applied to strings which have not yet been parsed into expressions.
By default, $Pre is not defined, meaning the input is evaluated directly:
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Save input expressions in held form:
Evaluation works as expected:
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Restore $Pre to its default:
The variable 
holds the inputs in unevaluated form:
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