subcommand of string that returns whether its string argument is a valid Tcl list

Accepts both the -strict and -failindex var options, with usual meanings. Note that the -strict is meaningless as the empty string is always a valid list.

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AMG: For Tcl 9, why not remove -strict for [string is list]? It is indeed meaningless. It's not even good for detecting empty lists, since any string consisting solely of whitespace is an empty list despite not being an empty string.

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