On 2013–08–24 Wolfgang Schuster wrote:

> I can’t reproduce your problem with the features key.

I was surprised that your example was running here, and I noticed
that a space makes all the difference:

  %% fails
  \definefontsynonym [Serif] [file:texgyrescholaregular] [features=default, fallbacks=cyrillic_regular]

  %% works
  \definefontsynonym [Serif] [file:texgyrescholaregular] [features=default,fallbacks=cyrillic_regular]
                                                                         ^^^

> To set the features for the fallback font I would use \
> definefontsynonym.

Makes more sense, yes.

Marco