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' ..."