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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 box_add(self, name, url, provider=None, force=False):
        '''
        Adds a box with given name, from given url.

        force: If True, overwrite an existing box if it exists.
        '''
        force_opt = '--force' if force else None
        cmd = ['box', 'add', name, url, force_opt]
        if provider is not None:
            cmd += ['--provider', provider]

        self._call_vagrant_command(cmd)