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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 _get_repo_dists_env(env):
    '''
    Get repo environment overrides dictionary to use in repo distributions process

    env
        A dictionary of variables to define the repository distributions
        Example:

        .. code-block:: yaml

            - env:
                - ORIGIN : 'jessie'
                - LABEL : 'salt debian'
                - SUITE : 'main'
                - VERSION : '8.1'
                - CODENAME : 'jessie'
                - ARCHS : 'amd64 i386 source'
                - COMPONENTS : 'main'
                - DESCRIPTION : 'SaltStack Debian package repo'

        .. warning::

            The above illustrates a common PyYAML pitfall, that **yes**,
            **no**, **on**, **off**, **true**, and **false** are all loaded as
            boolean ``True`` and ``False`` values, and must be enclosed in
            quotes to be used as strings. More info on this (and other) PyYAML
            idiosyncrasies can be found :ref:`here <yaml-idiosyncrasies>`.

    '''
    # env key with tuple of control information for handling input env dictionary
    # 0 | M - Mandatory, O - Optional, I - Ignore
    # 1 | 'text string for repo field'
    # 2 | 'default value'
    dflts_dict = {
        'OPTIONS': ('I', '', 'processed by _get_repo_options_env'),
        'ORIGIN': ('O', 'Origin', 'SaltStack'),
        'LABEL': ('O', 'Label', 'salt_debian'),
        'SUITE': ('O', 'Suite', 'stable'),
        'VERSION': ('O', 'Version', '9.0'),
        'CODENAME': ('M', 'Codename', 'stretch'),
        'ARCHS': ('M', 'Architectures', 'i386 amd64 source'),
        'COMPONENTS': ('M', 'Components', 'main'),
        'DESCRIPTION': ('O', 'Description', 'SaltStack debian package repo'),
    }

    env_dists = ''
    codename = ''
    dflts_keys = list(dflts_dict.keys())
    if env is None:
        for key, value in dflts_dict.items():
            if dflts_dict[key][0] == 'M':
                env_dists += '{0}: {1}\n'.format(dflts_dict[key][1], dflts_dict[key][2])
                if key == 'CODENAME':
                    codename = dflts_dict[key][2]
        return (codename, env_dists)

    if not isinstance(env, dict):
        raise SaltInvocationError(
            '\'env\' must be a Python dictionary'
        )

    env_man_seen = []
    for key, value in env.items():
        if key in dflts_keys:
            if dflts_dict[key][0] == 'M':
                env_man_seen.append(key)
                if key == 'CODENAME':
                    codename = value
            if dflts_dict[key][0] != 'I':
                env_dists += '{0}: {1}\n'.format(dflts_dict[key][1], value)
        else:
            env_dists += '{0}: {1}\n'.format(key, value)

    # ensure mandatories are included
    env_keys = list(env.keys())
    for key in env_keys:
        if key in dflts_keys and dflts_dict[key][0] == 'M' and key not in env_man_seen:
            env_dists += '{0}: {1}\n'.format(dflts_dict[key][1], dflts_dict[key][2])
            if key == 'CODENAME':
                codename = value

    return (codename, env_dists)