+static void insert_symbols(exset &es, const ex &e)
+{
+ if (is_a<symbol>(e)) {
+ es.insert(e);
+ } else {
+ for (const ex &sube : e) {
+ insert_symbols(es, sube);
+ }
+ }
+}
+
+static exset symbolset(const ex &e)
+{
+ exset s;
+ insert_symbols(s, e);
+ return s;
+}
+