Disambiguate the grammar for backtick.

The old syntax suggested that a trailing comma was OK inside backticks,
but in fact (due to ideosyncrasies of pgen) it was not.  Fix the grammar
to avoid the ambiguity.  Fred: you may want to update the refman.

Grammar/Grammar

@@ -80,7 +80,7 @@ arith_expr: term (('+'|'-') term)*
 term: factor (('*'|'/'|'%'|'//') factor)*
 factor: ('+'|'-'|'~') factor | power
 power: atom trailer* ['**' factor]
-atom: '(' [testlist] ')' | '[' [listmaker] ']' | '{' [dictmaker] '}' | '`' testlist '`' | NAME | NUMBER | STRING+
+atom: '(' [testlist] ')' | '[' [listmaker] ']' | '{' [dictmaker] '}' | '`' testlist1 '`' | NAME | NUMBER | STRING+
 listmaker: test ( list_for | (',' test)* [','] )
 lambdef: 'lambda' [varargslist] ':' test
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
@@ -100,3 +100,5 @@ argument: [test '='] test	# Really [keyword '='] test
 list_iter: list_for | list_if
 list_for: 'for' exprlist 'in' testlist_safe [list_iter]
 list_if: 'if' test [list_iter]
+
+testlist1: test (',' test)*

Include/graminit.h

@@ -64,3 +64,4 @@
 #define list_iter 319
 #define list_for 320
 #define list_if 321
+#define testlist1 322

Modules/parsermodule.c

@@ -844,6 +844,7 @@ VALIDATER(subscriptlist);       VALIDATER(sliceop);
 VALIDATER(exprlist);            VALIDATER(dictmaker);
 VALIDATER(arglist);             VALIDATER(argument);
 VALIDATER(listmaker);           VALIDATER(yield_stmt);
+VALIDATER(testlist1);
 
 #undef VALIDATER
 
@@ -1056,6 +1057,14 @@ validate_testlist(node *tree)
 }
 
 
+static int
+validate_testlist1(node *tree)
+{
+    return (validate_repeating_list(tree, testlist1,
+                                    validate_test, "testlist1"));
+}
+
+
 static int
 validate_testlist_safe(node *tree)
 {
@@ -2185,7 +2194,7 @@ validate_atom(node *tree)
             break;
           case BACKQUOTE:
             res = ((nch == 3)
-                   && validate_testlist(CHILD(tree, 1))
+                   && validate_testlist1(CHILD(tree, 1))
                    && validate_ntype(CHILD(tree, 2), BACKQUOTE));
             break;
           case NAME:
@@ -2671,6 +2680,9 @@ validate_node(node *tree)
           case testlist:
             res = validate_testlist(tree);
             break;
+          case testlist1:
+            res = validate_testlist1(tree);
+            break;
           case test:
             res = validate_test(tree);
             break;

Python/compile.c

@@ -2474,6 +2474,7 @@ com_assign(struct compiling *c, node *n, int assigning, node *augn)
 		
 		case exprlist:
 		case testlist:
+		case testlist1:
 			if (NCH(n) > 1) {
 				if (assigning > OP_APPLY) {
 					com_error(c, PyExc_SyntaxError,
@@ -2955,6 +2956,7 @@ is_constant_false(struct compiling *c, node *n)
 
 	case expr_stmt:
 	case testlist:
+	case testlist1:
 	case test:
 	case and_test:
 	case not_test:
@@ -3356,6 +3358,7 @@ get_rawdocstring(node *n)
 
 	case expr_stmt:
 	case testlist:
+	case testlist1:
 	case test:
 	case and_test:
 	case not_test:
@@ -3704,6 +3707,7 @@ com_node(struct compiling *c, node *n)
 	/* Expression nodes */
 	
 	case testlist:
+	case testlist1:
 	case testlist_safe:
 		com_list(c, n, 0);
 		break;
@@ -5447,6 +5451,7 @@ symtable_assign(struct symtable *st, node *n, int def_flag)
 		return;
 	case exprlist:
 	case testlist:
+	case testlist1:
 		if (NCH(n) == 1) {
 			n = CHILD(n, 0);
 			goto loop;