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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 buildmod(*modules):
    '''
    Build module using znc-buildmod

    CLI Example:

    .. code-block:: bash

        salt '*' znc.buildmod module.cpp [...]
    '''
    # Check if module files are missing
    missing = [module for module in modules if not os.path.exists(module)]
    if missing:
        return 'Error: The file ({0}) does not exist.'.format(', '.join(missing))

    cmd = ['znc-buildmod']
    cmd.extend(modules)
    out = __salt__['cmd.run'](cmd, python_shell=False).splitlines()
    return out[-1]