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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def render_pdf_file_to_image_files_pdftoppm_ppm(pdf_file_name, root_output_file_path, res_x=150, res_y=150, extra_args=None):
 """Use the pdftoppm program to render a PDF file to .png images. The root_output_file_path is prepended to all the output files, which have numbers and extensions added. Extra arguments can be passed as a list in extra_args. Return the command output."""


    if extra_args is None: extra_args = []

    if not pdftoppm_executable:
        init_and_test_pdftoppm_executable(prefer_local=False, exit_on_fail=True)

    if old_pdftoppm_version:
        # We only have -r, not -rx and -ry.
        command = [pdftoppm_executable] + extra_args + ["-r", res_x, pdf_file_name,
                                              root_output_file_path]
    else:
        command = [pdftoppm_executable] + extra_args + ["-rx", res_x, "-ry", res_y,
                                              pdf_file_name, root_output_file_path]
    comm_output = get_external_subprocess_output(command)
    return comm_output