human
Solve the following python programming problem given the following code:
def gatherHead(self) : 
    head = [] 
    def gather(testInstances) : 
        for test in testInstances : 
            if isinstance (test, list) : 
                head.extend (gather (test)) 
else : 
                head.append (test.head ()) 
gather (self.testInstances) 
    return filter (None, self) 

    
    

    Fix the buggy line: return filter (None, self)
assistant
#Here is Python code to fix the buggy line: return filter (None, self)
.

return filter (None, head)