trying to understand how these two forms of `pre` work in GF

PhonoPor.gf

resource PhonoPor = open Prelude in {

-- the implementation is wrong, the point is the use of `pre` 
  oper
    a : pattern Str = #("a") ;
    o : pattern Str = #("o") ;

    elisEm : Str ;
    {-- doesn't work, complains about "a" not being a pattern
    elisEm = pre {"em" ; "na" / "a" ; "no" / "o"} ;
    but this works: --}
    elisEm = pre {"em" ; "na" / a ; "no" / o} ;
    {-- this works
    elisEm = pre {
      "a" => "na" ;
      "o" => "no" ;
      _ => "em"
      } ;
    -- and so does this
    elisEm = pre {
      a => "na" ;
      o => "no" ;
      _ => "em"
      } ;
    --}
} ;