math mode in Staff.instrument

[Aurèle]

When I try to use some long math equations into Staff.instrument,
I get error: !Missing $ inserted
or a tex file which does'nt work with dvips
This worked with 2.1.28 version but not with 2.1.31

testfile.ly:
\version"2.1.31"
I = \notes { a1 b1 }

\score{
   \new Staff <<
   \set Staff.instrument = "$$\\alpha_a + \\alpha_b$$"
   \context Voice = "i" {\I}
   >>
   \paper{}
}

I checked into testfile.tex and I read:
...
\lyitem{ -6.6667 }{  7.5358 }{\magfontCRZGomMMBo\hbox{\alpha_b$$}}%
\lyitem{ -6.6667 }{  5.3803 }{\magfontCRZGomMMBo\hbox{+}}%
\lyitem{ -6.6667 }{  2.7803 }{\magfontCRZGomMMBo\hbox{$$\alpha_a}}%
...

Staff.instrument is truncated into small element, but the $$ (or $, i
have tested) math mode is not correctly placed.

How can I get a corrected form - whithout using $ before and after
each symbol?

Regards,
Aurèle Duda
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