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[an error occurred while processing this directive] $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.
  • See the Mathematica book: Section 2.13.1.
  • See also: $Post.