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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 check_dependencies():
 """Check external dependecies Return a tuple with the available generators. """

    available = []
    try:
        shell('ebook-convert')
        available.append('calibre')
    except OSError:
        pass
    try:
        shell('pandoc --help')
        available.append('pandoc')
    except OSError:
        pass
    if not available:
        sys.exit(error('No generator found, you cannot use md2ebook.'))
    check_dependency_epubcheck()
    return available