empty parens better than empty brackets

brag/brag.scrbl

@@ -708,7 +708,7 @@ A @deftech{pattern} is one of the following:
  
  @item{an explicit @deftech{sequence}: a @tech{pattern} surrounded by @litchar{(} and @litchar{)}.}
 
- @item{the @deftech{empty set}: a special @tech{pattern} that matches a list of zero tokens. When it appears on the right side of a rule, the empty set will match empty input (which obviously contains zero tokens), but also the ``gap'' between two existing tokens (which less obviously also contains zero tokens). The empty set can be denoted by @litchar{[]} (empty square brackets), @litchar{∅} (the Unicode empty-set character), or @litchar{Ø} (the slashed O).}
+ @item{the @deftech{empty set}: a special @tech{pattern} that matches a list of zero tokens. When it appears on the right side of a rule, the empty set will match empty input (which obviously contains zero tokens), but also the ``gap'' between two existing tokens (which less obviously also contains zero tokens). The empty set can be denoted by @litchar{()} (empty parentheses), @litchar{∅} (the Unicode empty-set character), or @litchar{Ø} (the slashed O).}
  ]
 
 

brag/examples/empty-symbol.rkt

@@ -1,6 +1,6 @@
 #lang brag
 
 top : xs | ys | zs
-xs : [] | "x" xs
+xs : () | "x" xs
 ys : Ø | "y" /ys
 zs : ∅ | "z" @zs

brag/rules/lexer.rkt

@@ -56,7 +56,7 @@
         (:or (:* (:or "\\\"" esc-chars (:~ "\"" "\\"))) "\\\\")
         "\"")
     (token-LIT (unescape-lexeme lexeme #\"))]
-   [(:or "[]" "Ø" "∅") (token-EMPTY lexeme)]
+   [(:or "()" "Ø" "∅") (token-EMPTY lexeme)]
    ["("
     (token-LPAREN lexeme)]
    ["["