keylget listvar ?key? ?retvar | {}?
Return the value associated with key from the keyed
list in the variable listvar. If retvar is not
specified, then the value will be returned as the
result of the command. In this case, if key is not
found in the list, an error will result.
If retvar is specified and key is in the list, then
the value is returned in the variable retvar and
the command returns 1 if the key was present within
the list. If key isn't in the list, the command
will return 0, and retvar will be left unchanged.
If {} is specified for retvar, the value is not
returned, allowing the Tcl programmer to determine
if a key is present in a keyed list without setting
a variable as a side-effect.
If key is omitted, then a list of all the keys in
the keyed list is returned.
This command is provided by Extended Tcl.