Q:

XSLT/Xpath: Why is it not possible to use the shortened notation for parent for the use attribute of the key function

It seems that it is not possible to use the shortened notation for parent for the use attribute of the key function, at least for my setup.
This does not work
<xsl:key name="eventsSuppressedByTheSameEventKey" match="suppressedMonitor" use="../suppressingMonitor/@event" />

But this does work
<xsl:key name="eventsSuppressedByTheSameEventKey" match="suppressedMonitor" use="parent::suppressingMonitor/@event" />

Why? Is the shortened notation not supported inside the use attribute for any reason?

A:

The short for: 
parent::suppressingMonitor/@event

is:
../@event

not:
../suppressingMonitor/@event

