Xpath how to select an element based on an order and dependent of its existence

how to select an element based on an order and dependent of its existence in XPath ? For example how to select the best quality video if it exist.

<VIDEOS>
    <LOW_RES>video_L.flv</LOW_RES>
    <HI_RES>video_H.flv</HI_RES>
    <HD/>
</VIDEOS>

this should return video_H.flv because the hd version doesn't exist

this case can exist (the videos names can be random):

<VIDEOS>
    <LOW_RES>video_L.flv</LOW_RES>
    <HI_RES>vi开发者_开发技巧deo_H.flv</HI_RES>
    <HD>video_hd.mp4</HD>
</VIDEOS>

this should return video_hd.mp4 because the hd version exist. Many thanks.

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Use:

/*/HD[text()]
|
 /*/HI_RES[text() and not(../HD/text())]
|
 /*/LOW_RES[text() and not(../HD/text()) and not(../HI_RES/text())]