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> > XML has:
> > 1. Omission of an optional element.
> > 2. Presence of an element having the attribute xsi:nil
> > 3. Presence of an element having empty content.
>
> I don't see how any of these three could be interpreted
> differently if the element content is defined to represent a number.
>
I've actually seen a system that did exactly that. The XML described the
output of a complex measuring device (think SCADA). The omission of the
optional element in an instance document (say, a temperature reading) meant
that the device didn't have a sensor to measure that. Including the element
in an instance document with the nil attribute meant that the device had the
sensor but it was not reporting (turned off, malfunctioning, etc.) Including
the element with empty content meant that the device had the sensor, it was
(assumed to be) working, but didn't have an output for that instance
document.
Whether or not you think this is a good design, it's clearly an example of
an XML application taking advantage of those distinctions for element
content defined to represent a number.
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