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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 add_comment(issue_key,
                comment,
                visibility=None,
                is_internal=False,
                server=None,
                username=None,
                password=None):
    '''
    Add a comment to an existing ticket. Return ``True`` when it successfully
    added the comment.

    issue_key
        The issue ID to add the comment to.

    comment
        The body of the comment to be added.

    visibility: ``None``
        A dictionary having two keys:

        - ``type``: is ``role`` (or ``group`` if the JIRA server has configured
          comment visibility for groups).
        - ``value``: the name of the role (or group) to which viewing of this
          comment will be restricted.

    is_internal: ``False``
        Whether a comment has to be marked as ``Internal`` in Jira Service Desk.

    CLI Example:

    .. code-block:: bash

        salt '*' jira.add_comment NE-123 'This is a comment'
    '''
    jira_ = _get_jira(server=server,
                      username=username,
                      password=password)
    comm = jira_.add_comment(issue_key,
                             comment,
                             visibility=visibility,
                             is_internal=is_internal)
    return True