Experimental command provided by [NRE], available in HEAD and onwards from the release of Tcl8.6a2.

The command provides proper tailcalls, with the following semantics:

   * a proc/lambda invokes '''[[tailcall foo [bar] $soom]]'''
   * '''bar''' is invoked and the value of '''soom''' fetched, as if '''[[list foo [bar] $soom]]''' had been called
   * a command named '''foo''' is looked up ''in the current context''
   * the current proc/lambda replaces itself in the call stack with a call to the just found command

That is: '''tailcall foo [bar] $soom''' is very similar to '''return [uplevel 1 [list foo [bar] $soom]]''' with two exceptions:
   1. '''foo''' is looked up in the ''current'' context, not in the caller's
   1. the stack frame is ''really'' gone, not just ''virtually''. This has positive effects on memory, and a possibly confusing effect on stack traces.

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%| [Category Concept]  |  [Category Control Structure]  |  [Category Functional Programming] |%
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