sclass VarContext {
  VarContext parent;
  MapSO vars;
  
  *() {}
  *(VarContext *parent) {}
  
  O get(S name) {
    if (containsKey(vars, name))
      ret mapGet(vars, name);
    if (parent != null)
      ret parent.get(name);
    null;
  }
  
  void set(S name, O value) {
    vars = mapPutOrCreate(vars, name, value);
  }
}