Grammar Specification Question

When I was reading the speech grammar specification located at: http://www.w3.org/TR/grammar-spec/
 
I read the comment on recursion stated as follows:

The Speech Recognition Grammar Specification has the expressive power of a Context Free Grammar. This arises because the language permits a rule to directly or indirectly reference itself. [Note: a Conforming XML Form Grammar Processor or Conforming ABNF Form Grammar Processor is not required to support recursive grammars.]

Why is recursion not required for a grammar processor to conform? Is this a difficult feature to add? Do some processors support this feature already? Any information that can be provided on this decision would be extremely helpful for my development project. Thank you in advance.

-Cassandra Guy



		
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