There are five hidden symbols in math, which are not accessible through the UI at the moment:
They are defined here: starmath/source/commands.src
It seems they could be easily added like done in 8e0aec74cadcc7ad0fe38d52f542c6a6643308ae.
Created attachment 89201 [details]
Testdoc with the functions
Here's a test document, which shows, that those functions are interpreted by math.
there are all symbols from descripion in Element dock in Math in LO 6.3