truncated math mode in Staff.instrument

[Aurèle]

Using lily2.1.31 on XP/cygwin

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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www.musiquefrancaise.asso.fr