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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 pymux_key_to_prompt_toolkit_key_sequence(key):
 """ Turn a pymux description of a key. E.g. "C-a" or "M-x" into a prompt-toolkit key sequence. Raises `ValueError` if the key is not known. """

    # Make the c- and m- prefixes case insensitive.
    if key.lower().startswith('m-c-'):
        key = 'M-C-' + key[4:]
    elif key.lower().startswith('c-'):
        key = 'C-' + key[2:]
    elif key.lower().startswith('m-'):
        key = 'M-' + key[2:]

    # Lookup key.
    try:
        return PYMUX_TO_PROMPT_TOOLKIT_KEYS[key]
    except KeyError:
        if len(key) == 1:
            return (key, )
        else:
            raise ValueError('Unknown key: %r' % (key, ))