TraceOriginal

is an option for Trace that specifies whether to test the form of each expression before its head and arguments are evaluated.

Details

With the default TraceOriginal->False, the forms of expressions generated during an evaluation are tested only after their head and arguments have been evaluated. In addition, evaluation chains for expressions which do not change under evaluation are not included.
With TraceOriginal->True, the forms before evaluation of the head and arguments are also tested, and evaluation chains for expressions which do not change under evaluation are included.
With TraceOriginal->Automatic, the forms before evaluation of the head and arguments are also tested, but evaluation chains for expressions that do not change under evaluation are not included.

Trace evaluation of an expression showing evaluation chains for expressions that change:

Show evaluation chains even for expressions that do not change:

Show the starting point of each evaluation chain, but drop chains that do not change:

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