+/*
+ * Test tags for some forbidden cases.
+ *
+ * return: NULL if OK,
+ * explanatory message otherwise.
+ */
+
+static const char *
+illegal_tag (const char *tag, notmuch_bool_t remove)
+{
+
+ if (*tag == '\0' && ! remove)
+ return "empty tag forbidden";
+
+ /* This disallows adding the non-removable tag "-" and
+ * enables notmuch tag to take long options more easily.
+ */
+
+ if (*tag == '-' && ! remove)
+ return "tag starting with '-' forbidden";
+
+ return NULL;
+}
+