use this one /descendant::head[parent::section] Can you provide a
general rule?

It is best not to even try, The processor is mandated to do this rewrite
and it can rewrite it using internal facilities not available in the top
level syntax if it wishes. So it is better to write a simple clear xpath
expression that the each processor optimiser can write to an optimal
form _for that processor_ rather than try to second guess the
optimisation strategy and do the rewrites by hand.

Going back to the beginning of time we used to tell people that that was
an expensive full document search and it is better to go /a/b/c/foo if
you know all the foo elements are at that level, to save the processor
searching arbitrarily deep. But now some database backed processors can
map //foo to a pre-computed index so it is fast but /a/b/c/foo does an
iterative walk over that level of the document so is relatively slow.

________________________________________________________________________
The Numerical Algorithms Group Ltd is a company registered in England
and Wales with company number 1249803. The registered office is:
Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.