Is "(//c)[a]" valid in Xpath 1.0?

Hello,

  I sometimes write XPath 1.0 expressions like

(//c)[1]

  It works fine (at least in libxml).

  But now I've realized that I can't find appropriate BNF productions
for parsing such expressions. Am I missed something? Are such
expressions valid in XPath 1.0?


  Regards, Oleg