Add grouping operators for ebnf

extra/peg/ebnf/ebnf-tests.factor

@@ -52,3 +52,16 @@ IN: temporary
 } [
   "one two | three" 'choice' parse parse-result-ast
 ] unit-test
+
+{
+  T{ ebnf-sequence f
+     V{ 
+       T{ ebnf-non-terminal f "one" }
+       T{ ebnf-choice f
+          V{ T{ ebnf-non-terminal f "two" } T{ ebnf-non-terminal f "three" } }
+       }
+     }
+  } 
+} [
+  "one (two | three)" 'choice' parse parse-result-ast
+] unit-test

extra/peg/ebnf/ebnf.factor

@@ -95,8 +95,16 @@ DEFER: 'rhs'
 : 'element' ( -- parser )
   'non-terminal' 'terminal' 2array choice ;
 
+DEFER: 'choice'
+
+: 'group' ( -- parser )
+  "(" token sp hide
+  [ 'choice' sp ] delay
+  ")" token sp hide 
+  3array seq [ first ] action ;
+
 : 'sequence' ( -- parser )
-  'element' sp 
+  'element' sp 'group' sp 2array choice 
    repeat1 [ 
      dup length 1 = [ first ] [ <ebnf-sequence> ] if
    ] action ;