Several [Tcl] commands, including [expect] and [switch], take a '--'.  [RS] calls this,
"the switch to end all switches".  Here's why:

What is the difference between

    switch $variable {...
and
    switch -- $variable {...

?  None--mostly; if, however, $variable contains a string such as "-glob", then the
first form will be interpreted as 

    switch -glob {...

Do you see the problem?  Do you see how your datum might be interpreted as a command
"switch"?  In casual English words, the '--' says, "treat everything that follows like
data, '''not''' as a 'switch' or 'flag' ..."

Alas, not every tcl command which takes a switch/flag recognizes -- as a valid entry. This means that the casual tcl programmer needs to check regularly to see if the command being coded accepts the -- or not. It surely would be nice if all tcl commands which take a -argument also accepted -- ...
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[Category Syntax]