Hi Jeni,

>  
>  I imagine that a processor would be able to spot situations where the
>  position() or last() function had been called and only compose the
>  steps that were composable.
>    

It seems to me obviously not so -- I mean the general task of spotting ***any***
function in the expression, that could reference not only the specific item in the
sequence. This includes any user-defined functions.

This leads us to the great topic of type-checking and why it is necessary...

In case there isn't strong type-checking such re-writing/optimisations are
impossible in the general case.


Cheers,
Dimitre.

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