+/site/open_auctions/open_auction[(not(bidder/following::bidder) or not(bidder/preceding::bidder)) and (bidder/following::bidder and bidder/preceding::bidder)]/interval
+/site/open_auctions/open_auction[(not(bidder/following::bidder) or not(bidder/preceding::bidder)) or (bidder/following::bidder and bidder/preceding::bidder)]/interval
+/site/open_auctions/open_auction[number(bidder[1]/increase) < number(bidder[floor((last() + 1) div 2)]/increase) and number(bidder[floor((last() + 1) div 2)]/increase) < number(bidder[last()]/increase)]/interval