def get_free_symbols(s, symbols, free_symbols=None):
    """
    Returns free_symbols present in `s`.
    """
    free_symbols = free_symbols or []
    if not isinstance(s, list):
        if s in symbols:
            free_symbols.append(s)
        return free_symbols

    for i in s:
        free_symbols = get_free_symbols(i, symbols, free_symbols)

    return free_symbols