Creates a temporary file and returns an open read-write channel to it.

    :   '''[file] tempfile''' ?''varName''? ?''template''?

If ''varName'' is given, set that variable to the name of the temporary file (otherwise tries to delete the file at the first opportunity). If ''template'' is given, use it to guide the filename generated.

This command was introduced by [TIP]#210 in Tcl 8.6.
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%| [Category Command] | [Category File] |%
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