A quick word about the notation used to describe Tcl commands. In general, a description of a command is the name of the command followed by its arguments separated by spaces. An example is:
set varName ?value?
which is a description of the Tcl set command, which takes a variable name varName and an optional argument, a value.
Optional arguments are enclosed in question mark, ?, pairs, as in the example.
A series of three dots ... represents repeated
arguments. An example is a description of the unset
command:
unset varName ?varName varName ...?
which shows that the unset command has at least one compulsory
argument varName but has any number of subsequent optional
arguments.