cosmetic (adds text-file: list of vlkb-extensions over SIAv2)

siav2-vlkb-extensions.tex

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+
+VLKB extension above standard SIAv2 has:
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+* overlap code
+* reconstructs full URLs for cutout and merge
+* response repeats all input params to the query
+* adds subsurvey metadata
+* groups the results by (sub)surveys
+* adds mergeable (virtual) datasets
+