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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 default(ruby=None, runas=None):
    '''
    Returns or sets the currently defined default ruby

    ruby
        The version to set as the default. Should match one of the versions
        listed by :py:func:`rbenv.versions <salt.modules.rbenv.versions>`.
        Leave blank to return the current default.

    CLI Example:

    .. code-block:: bash

        salt '*' rbenv.default
        salt '*' rbenv.default 2.0.0-p0
    '''
    if ruby:
        _rbenv_exec(['global', ruby], runas=runas)
        return True
    else:
        ret = _rbenv_exec(['global'], runas=runas)
        return '' if ret is False else ret.strip()