[Ur] Abstract type for 'id' attributes?

[Adam Chlipala]

Currently, Ur/Web supports an 'id' attribute on any XML element.  I 
added this when someone requested support for the <label> tag.

Currently, 'id' attributes have type [string].  This freaks me out 
somewhat, since it's anti-modular; different functions written by 
different people might need to coordinate to avoid duplicating IDs.  The 
use of IDs is convenient for FFI JavaScript code that wants to get 
handles to DOM nodes.

So, I'm thinking of adding an abstract type of IDs, with only one 
built-in operation: "generate fresh ID."  Every tag's 'id' attribute 
would change to take a value of this type.  Under the hood, these would 
just be strings, making for easy FFI integration.

Does anyone object to the idea?  Any other feedback?