fix: allow both "/=" and "=/"

src/parser.cxx

@@ -146,7 +146,7 @@ static auto parse_impl(std::istream & in) {
     if (token != name) { push_rule(); }
     name = token;
     in >> token; // = OR /=
-    if ((is_one_of = (token == "/="))) { rule = store[name]; }
+    if ((is_one_of = (token == "/=" || token == "=/"))) { rule = store[name]; }
   }
   push_rule();