Recursive function with a parameter

[MOH]

Hey,

As a homework, I have to define a function that returns a frozenset of the positions who are going to explode (the function is_to_explode() is already defined and works). So what this function does, is, given a board, (defined as a list), I’d have to check each elements to know if in this position the element will explode, and that by using a recursive way, well I just don’t see a startpoint. I wrote a function, but actually it doesn't work and I don't see why.

Well if anyone could help, it’d be really great

Thanks in advance.
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def get_all_positions_to_explode(board,start_pos=(1,1)):
    """

        RECURSIVE !!!

        Return a frozen set of all positions on the given board that
        have a disk that satisfies the conditions to explode, starting
        from the given position and proceeding to the top of the board
        using the next function.
        - The function returns the empty set if the given start position
          is None.
        ASSUMPTIONS
        - The given board is a proper board.
        - The given start position is either None or it is a proper position
          for the given board.
        NOTE
        - The second parameter should not be included in the code that
          is given to the students. They must learn to extend functions
          with extra parameters with a default value. The documentation
          of the function must be changed in view of that.
    """

    lst = set()

    if start_pos[1]> len(board):
        return frozenset(lst)

    for col in range(1,len(board[1])):

        pos = (col,start_pos[0])
        if is_to_explode(board,pos)==True:
            set.add(lst,pos)

    return get_all_positions_to_explode(board,(1,start_pos[1]+1))