Returns URL that represents the given DOI. The function is installed from the hslookup plugin which uses http://www.handle.net/.
Note that you need to have in your Virtuoso database ini file in section Plugins added the hslookup.dll file, which location should be in the plugins folder under your Virtuoso server installation. For ex:
[Plugins] LoadPath = ./plugin ... Load6 = plain,hslookup
A string of URL that represents the given DOI.