<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def authorization_middleware(environ, start_response=None):
    '''Parse the ``HTTP_AUTHORIZATION`` key in the ``environ``.

    If available, set the ``http.authorization`` key in ``environ`` with
    the result obtained from :func:`~.parse_authorization_header` function.
    '''
    key = 'http.authorization'
    c = environ.get(key)
    if c is None:
        code = 'HTTP_AUTHORIZATION'
        if code in environ:
            environ[key] = parse_authorization_header(environ[code])