def flatten_list(ls=[]):
    flattened_list = []
 
    def aux(ls):
        for elem in ls:
            if not isinstance(elem, list):
                print("ADDING NO-LIST ELEMENT...")
                flattened_list.append(elem)
            else:
                print("RECURSION...")
                aux(elem)
 
    aux(ls)
 
    return flattened_list
 
 
 
print(flatten_list([[1,2,3,[4,5]],[6,7,8,9],10]))
				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
				
				
				
				
				
			 
			
				
			
			
				
	
		
		
	
	
		RECURSION...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
RECURSION...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
RECURSION...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
ADDING NO-LIST ELEMENT...
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]