bool try_retrieve (SCM key, SCM *val);
bool contains (SCM key) const;
void set (SCM k, SCM v);
SCM get (SCM k) const;
void remove (SCM k);
bool try_retrieve (SCM key, SCM *val);
bool contains (SCM key) const;
void set (SCM k, SCM v);
SCM get (SCM k) const;
void remove (SCM k);