change argument syntax to use $ instead of {}

parser/parser.cpp

@@ -62,7 +62,7 @@ struct key_path : list< seq< sor< alpha, one< '_' > >, star< sor< alnum, one< '_
 
 // argument
 struct argument_index : plus< digit > {};
-struct argument : seq< one< '{' >, must< argument_index, one< '}' > > > {};
+struct argument : seq< one< '$' >, must< argument_index > > {};
 
 // expressions and operators
 struct expr : sor< dq_string, sq_string, number, argument, true_value, false_value, key_path > {};

parser/test.cpp

@@ -30,8 +30,8 @@ static std::vector<std::string> valid_queries = {
     "_ = a",
     "_a = _.aZ",
     "a09._br.z = __-__.Z-9",
-    "{0} = {19}",
-    "{0} = {0}",
+    "$0 = $19",
+    "$0=$0",
 
     // operators
     "0=0",
@@ -75,10 +75,10 @@ static std::vector<std::string> invalid_queries = {
     "- = a",
     "a..b = a",
     "a$a = a",
-    "{} = {0}",
-    "{-1} = {0}",
-    "{a} = {0}",
-    "{ = }",
+    "{} = $0",
+    "$-1 = $0",
+    "$a = $0",
+    "$ = $",
 
     // operators
     "0===>0",