human
Solve the following python programming problem given the following code:
def __str__(self) : 
    '
        Return generated XML representing the stored element tree.
        ' 
    def to_xml(obj) : 
        element = doc.createElement (obj._name) 
        for k in obj._attributes : 
            element.setAttribute (k, str (obj._attributes [k])) 
for e in obj._elements : 
            element.appendChild (to_xml (e)) 
return element 
doc = xml.dom.minidom.Document () 
    doc.appendChild (to_xml (to_xml)) 
    return re.sub ('<\\?xml.*\\?>', '', doc.toprettyxml (indent = '  ')).strip () 

    
    

    Fix the buggy line: doc.appendChild (to_xml (to_xml))
assistant
#Here is Python program to fix the buggy line: doc.

doc.appendChild (to_xml (self))