default {
		    return -code error -errorcode {CMDR SET UNKNOWN OPTION} \
			"Unknown option \"$o\", expected -extend"
		}
	    }
	}
	if {[llength $args] != 1} {
	    # The method of getting the command name is so complicated
	    # to account for calls from specifications, where this is
	    # aliased as 'common'.
	    set cmd [lindex [dict get [info frame -1] cmd] 0]
	    return -code error -errorcode {CMDR SET WRONG-ARGS} \
		"Expected one argument."
		"wrong # args: should be \"$cmd key ?-extend? data\""
	}
	set data [lindex $args 0]
	if {$extend} {
	    set data [my get $key]$data
	}
	dict set mystore $key $data
	return