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:
Restore $Pre to its default:
Introduced in 1988