human
Solve the following python programming problem given the following code:
def _print_Sum(self, expr) : 
    def _xab_tostr(xab) : 
        if (len (xab) == 1) : 
            return self._print (xab [0]) 
else : 
            return self._print (((xab [0],) + tuple (xab [1 :]))) 
L = ', '.join ([_xab_tostr (l) for l in expr.limits]) 
    return ('Sum(%s, %s)' % (self._print (_xab_tostr.function), L)) 

    
    

    Fix the buggy line: return ('Sum(%s, %s)' % (self._print (_xab_tostr.function), L))
assistant
#Here is Python program to fix the buggy line: return ('Sum(%s, %s)' % (self.

return ('Sum(%s, %s)' % (self._print (expr.function), L))