Re: references and dependency

>
> By the way, it is certainly possible to improve on the above. For example,
> allowing MIPs to be part of the dependency graph would be useful.
>
> Can you give an example?
>

Currently validation follows calculations including computing required,
relevant, and readonly. Without changing that, here is an example where the
validity of a node could depend on its relevance using the `relevant()`
function:

    <xf:bind ref="b" relevant="../a = 42"/>
    <xf:bind ref="c" constraint="relevant(../b)"/>

Further, if recalculate and revalidate were unified, the opposite might be
possible:

    <xf:bind ref="b" constraint=". = ../a"/>
    <xf:bind ref="c" relevant="valid(../b)"/>

This can create cycles, which would have to be detected.

-Erik