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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 find_users_by_email_starting_with(email_prefix=None, cursor=None, page_size=30):
 """ Returns a command that retrieves users by its email_prefix, ordered by email. It returns a max number of users defined by page_size arg. Next result can be retrieved using cursor, in a next call. It is provided in cursor attribute from command. """

    email_prefix = email_prefix or ''

    return ModelSearchCommand(MainUser.query_email_starts_with(email_prefix),
                              page_size, cursor, cache_begin=None)