jobname in luamplib

In plain MP with "mpost", the internal variable "jobname" resolves to the stem of the name of the current source file.

But if I use "luamplib" with "lualatex" it always resolves to "mpout" -- would it be possible to propagate the current source filename from the lualatex level down to the MP level please?

Or is there an alternative approach with lualatex?

thanks, Toby Thurston
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http://tug.org/metapost/