append-widget: make function! [
	"Append a custom widget to widgets context."
	spec [block!] "Widget spec"
	/local word
] [
	;	Is widget word already in use?
	all [
		find words word: to word! first spec
		gui-error reform [word "is already in use"]
	]
	;	append widget to widgets context
	widgets: make widgets spec
	;	Is widget derived from another widget?
	all [
		find words third spec
		widgets/:word/type: third spec
	]
	;	register new widget
	insert tail words word
]