[selectors] what is the combinator '/' IDENT '/'

Hello,

in "http://dev.w3.org/csswg/selectors/#grammar", the production for 
combinator reads:

combinator
     /* combinators can be surrounded by whitespace */
     : S+ | S* [ '>' | '+' | '~' | COLUMN | '/' IDENT '/' ] S*
     ;

Just wondering what the branch '/' IDENT '/' stands for. There seems to 
be no description for it.