def check_for_overlap(a, b):
    """ Returns true if two sets are not overlaping. Used here to check if two stems share a common residue. 
        If they do, returns False.    
    """
    # https://stackoverflow.com/questions/3170055/test-if-lists-share-any-items-in-python
    # return True is the is no overlap, else False
    return(set(a).isdisjoint(b))