Re: [patch] add "underwave" face attribute

[Juri Linkov]

> (:underline
>  (choice :tag "Underline"
>          :help-echo "Control text underlining."
>          (const :tag "Off" nil)
>          (list :tag "On"
>          (const :format "" :value :color)
>          (choice :tag "Color" (const :tag "Foreground Color" 
> foreground-color) color)
>                (const :format "" :value :style)
>                (choice :tag "Style"
>                        (const :tag "Line" line)
>                        (const :tag "Wave" wave)))))

Very nice, we could use it for `flyspell', `flymake', `next-error'
to highlight errors, etc.

But what to do with platforms where it's not yet supported?
There should be a way to detect its availability
and to fall back to other face attributes.

Currently `:underline' can test its support with `(supports :underline t)'.

Maybe `underwave' should do this with something like:

(defface underwave
  '((((supports :underline wave))
     :underline (:style wave :color "red"))
    (t
     :underline (:style line :color "red")))
  "Basic underwave face."
  :group 'basic-faces)