Kernel.SpecialForms.__STACKTRACE__

You're seeing just the macro __STACKTRACE__, go back to Kernel.SpecialForms module for more information.
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__STACKTRACE__

View Source (macro) (since 1.7.0)

Returns the stacktrace for the currently handled exception.

It is available only in the catch and rescue clauses of try/1 expressions.

To retrieve the stacktrace of the current process, use Process.info(self(), :current_stacktrace) instead.