--- a/dav/sync.h	Wed May 30 16:51:52 2018 +0200
+++ b/dav/sync.h	Wed May 30 18:04:43 2018 +0200
@@ -112,7 +112,7 @@
 int cmd_empty_trash(CmdArgs *args);
 
 /**
- * filter ::= tag_list | (operator , ("(" , filter , ")")*)
+ * filter ::= operator? , (tag_list | ("(" , filter , ")")+)
  * tag_list ::= tag , ("," tag)*
  * operator ::= "&" | "|" | "1" | "0"
  */