[Ur] error reporting

[Gian Perrone]

On 2/11/11 11:49 PM, Adam Chlipala wrote:
>
> This is the behavior I meant to implement (though I don't have a solid 
> story on 0-vs.-1-based numbering for the two dimensions!). 
This appears to be almost universally standard.

> Think of the first position as pointing to the first character in 
> question and the second position as pointing right _after_ the last 
> character in question.  Among other properties, this scheme makes it 
> possible to distinguish an empty region from a 1-character region.
>
As does this.

--Gian