def list_clusters(nextToken=None, maxResults=None):
    """
    Returns a list of existing clusters.
    See also: AWS API Documentation
    
    Examples
    This example lists all of your available clusters in your default region.
    Expected Output:
    
    :example: response = client.list_clusters(
        nextToken='string',
        maxResults=123
    )
    
    
    :type nextToken: string
    :param nextToken: The nextToken value returned from a previous paginated ListClusters request where maxResults was used and the results exceeded the value of that parameter. Pagination continues from the end of the previous results that returned the nextToken value. This value is null when there are no more results to return.
            Note
            This token should be treated as an opaque identifier that is only used to retrieve the next items in a list and not for other programmatic purposes.
            

    :type maxResults: integer
    :param maxResults: The maximum number of cluster results returned by ListClusters in paginated output. When this parameter is used, ListClusters only returns maxResults results in a single page along with a nextToken response element. The remaining results of the initial request can be seen by sending another ListClusters request with the returned nextToken value. This value can be between 1 and 100. If this parameter is not used, then ListClusters returns up to 100 results and a nextToken value if applicable.

    :rtype: dict
    :return: {
        'clusterArns': [
            'string',
        ],
        'nextToken': 'string'
    }
    
    
    :returns: 
    (string) --
    
    """
    pass
