Use balanced braces for doxygen formulas.

[bangerth]

doxygen used to require that formulas be written as

  @f{align*}
  ...
  @f}

which is, for OCD people like all of us, unsettling because the braces are not balanced. But for several years already, it now allows the alternative syntax

  @f{align*}{
  ...
  @f}

which is far more satisfying. A little bit of PERL magic makes that possible.