Re: use-when attribute?

In short, it isn't hard for a system to evaluate this expression in a single
scan, or perhaps three scans, and there is no need to allocate any memory
for temporary results.

Even if the result of //a|//b|//c is assigned to a variable and you then do
count() on the variable, the system may be able to tell that it doesn't need
to allocate memory to hold the value (it might be able to see that it's only
used once).

The main reason the key is slower in this case is that you are only using it
once. But there's no good reason for ever using a key with a fixed use
expression more than once, because you could always put the result of the
first evaluation in a variable. So yes, I can't see any reason for using
this construct.

Cheers,
Geert