code: @Override
    public void fillAssignedKeys(Set<String> keys)
    {
      TreeSet<String> subKeys = new TreeSet<>();
      
      for (ExprKraken expr : _exprs) {
        subKeys.clear();
        
        expr.fillAssignedKeys(subKeys);
        
        keys.addAll(subKeys);
      }
    }
docs: Returns all the keys assigned statically, e.g. by param or literal.