[Corpora-List] Use of dash for "to be" elision in russian

[Galibert Olivier]

  Hello corpora list members,

Russian has the interesting characteristic of representing an elided instance of "to be" with a dash in specific cases.  See for instance the last sentence of:
http://en.wikibooks.org/wiki/Russian/Grammar/Articles
to see what I"m talking about.

My problem is to know whether that dash is grammatically required or optional.  Depending on my sources I see both opinions.  Does anyone know for sure, or maybe it's a regional issue?

Best,

  Olivier Galibert

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