XQuery Puzzle

Sorry for asking an XQuery question here, if there is a more
appropriate forum please let me know.

Anyway, a question relating to this simple XQuery has been circulating
on another group, but I am puzzled as to whether using // on a
sequence is valid (see the return clause below). I tried in a couple
of XQuery parsers (Saxon9 + the one built into XML Spy) and both
return no results ??

let $set := (
    <div1>one   </div1>,
    <div1 class="doc">two   </div1>,
    <div1>three   </div1>,
    <div1 class="doc">four </div1>,
    <gotcha>START</gotcha>,
    <div1>five   </div1>
)
return $set//gotcha/preceding-sibling::div1[@class="doc"][1]