Relax NG: Drop down list at attribute with value domain

[Metin]

Hi,

It would be nice, analogous to element and attribute selection, to get a drop-down list at attribute values that can be only one of the specified values in the Relax NG grammar.

Example grammar-piece:

example = element Example
{
attribute valueDomain { "value1" | "value2" | "value3"}
}

Example xml:
<Example valueDomain="<!-- at this point I'd like to see value1, value2 and value3 in a drop-down list -->" />

Regards,
Metin