fix Tset's tagset-aware string representation

libwccl/values/tset.cpp

@@ -12,7 +12,7 @@ std::string TSet::to_raw_string() const
 
 std::string TSet::to_string(const Corpus2::Tagset& tagset) const
 {
-	return tagset.tag_to_symbol_string(tag_);
+	return "{" + tagset.tag_to_symbol_string(tag_) + "}";
 }
 
 std::string TSet::var_repr(const std::string &var_name)