code: public Set getKeys(String fqn) {
          Node n=findNode(fqn);
          Map data;
  
          if(n == null) return null;
          data=n.getData();
          if(data == null) return null;
          return data.keySet();
      }
docs: Gets the keys of the {@code data} map. Returns all keys as Strings. Returns null if node
  does not exist.
  @param fqn The fully qualified name of the node
  @return Set A set of keys (as Strings)