Fix #3. Use { and } for explicit grouping

fluent.asdl

@@ -9,14 +9,16 @@ module Fluent
 
     pat = Pattern(expr* elements, bool quoted)
 
-    expr = MessageReference(iden id)
-         | ExternalArgument(iden id)
-         | CallExpression(iden callee, expr* args)
-         | SelectExpression(expr exp, mem* vars)
-         | MemberExpression(expr obj, memkey key)
-         | KeyValueArgument(iden name, argval val)
-         | Number(string value)
-         | String(string value)
+    expr = Selector(sel)
+         | SelectExpression(sel sel, mem* vars)
+
+    sel = MessageReference(iden id)
+        | ExternalArgument(iden id)
+        | CallExpression(iden callee, expr* args)
+        | MemberExpression(expr obj, memkey key)
+        | KeyValueArgument(iden name, argval val)
+        | Number(string value)
+        | String(string value)
 
     mem = Member(memkey key, pat value, bool default)
     memkey = Number(string value)

fluent.ebnf

@@ -32,15 +32,17 @@ quoted-text          ::= ([^{"] | '\{' | '\"')+
 block-text           ::= NL __ '|' unquoted-pattern
 
 placeable            ::= '{' __ expression __ '}'
-expression           ::= quoted-pattern
+expression           ::= selector-expression | select-expression
+
+selector-expression  ::= quoted-pattern
                        | number
                        | identifier
                        | variable
-                       | select-expression
                        | member-expression
                        | call-expression
+                       | placeable
 
-select-expression    ::= expression __ ' ->' __ variants-list
+select-expression    ::= selector-expression __ ' ->' __ variants-list
 member-expression    ::= identifier '[' keyword ']'
 call-expression      ::= builtin '(' __ arglist? __ ')'
 arglist              ::= argument (__ ',' __ arglist)?