2004 Add xsl:select instruction - #2008

To showcase the utility of the instruction, I recommend including a brief example with CDATA, e.g.,

<![CDATA[
    let $anchor := following-sibling::anchor[1]
    return
         following-sibling::*[. << $anchor]
    (: Get all elements after the context that precede the anchor. :)
]]>

The operators << and >> are easily missed, hence both the example and its internal explanation.

At meeting 122, the CG agreed to merge this PR.