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 is not 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.