+ assert_return(m, -EINVAL);
+ assert_return(m->sealed, -EPERM);
+ assert_return(type != 0 || !contents, -EINVAL);
+
+ if (type == 0 || !contents) {
+ const char *cc;
+ char tt;
+
+ /* Allow entering into anonymous containers */
+ r = sd_bus_message_peek_type(m, &tt, &cc);
+ if (r <= 0)
+ return r;
+
+ if (type != 0 && type != tt)
+ return -ENXIO;
+
+ if (contents && !streq(contents, cc))
+ return -ENXIO;
+
+ type = tt;
+ contents = cc;
+ }