def get_pieces(struct, splits):
    """Breaks up the structure at fixed positions, returns the pieces.
    
    struct: structure string in sprinzl format
    splits: list or tuple of positions to split on

    This is a helper function for the sprinzl_to_vienna function.
    
    struct = '...===...===.'
    splits = [0,3,7,-1,13]
    pieces -> ['...','===.','..===','.']
    """
    pieces = []
    for x in range(len(splits)-1):
        pieces.append(struct[splits[x]:splits[x+1]])
    return pieces