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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_client(site=None):
 """Get a citrination client"""

    if 'CITRINATION_API_KEY' not in environ:
        raise ValueError("'CITRINATION_API_KEY' is not set as an environment variable")
    if not site:
        site = environ.get("CITRINATION_SITE", "https://citrination.com")
    return CitrinationClient(environ['CITRINATION_API_KEY'], site)