[XPath] Simplified grouping in primary expressions

Hi,

  I suggest to extend Primary expressions in XPath 2.0 to allow a subset
of the an+b expression as defined in

  http://www.w3.org/TR/css3-selectors/#nth-child-pseudo

for simplified grouping. It is basically possible to express the same
functionality using count(...), the following-/preceding-sibling axes
and the mod operator but this is typically rather unconvenient and hard
to understand for beginners. It should for example be possible to use

  tr[2n]

to select all even tr children of the context node.

regards.