[Html-widget] Either/Or validation

[Will Hawes]

Probably I've missed something, but how do you go about implementing
either/or validation?

I have a select box and a set of text fields and require either a
selected option in the select box or the text fields to be filled in,
but not both. It occurred to me to use ::Constraint::Callback, but
this doesn't appear to provide access to the values of other fields.

Writing a custom constraint won't be a problem, just making sure I've
not overlooked an existing way to do this.

Thanks, Will