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' ..." ---- [Category Syntax]