'Install everything in this set (after having downloaded and unpacked
the packages)'

def install(self, install_options, global_options=(), *args, **kwargs):

    to_install = self._to_install()
    if to_install:
        logger.info('Installing    collected    packages:    %s', ',    '.join([req.name for req in to_install]))
    with indent_log():
        for requirement in to_install:
            if requirement.conflicts_with:
                logger.info('Found    existing    installation:    %s', requirement.conflicts_with)
                with indent_log():
                    requirement.uninstall(auto_confirm=True)
            try:
                requirement.install(install_options, global_options, *args, **kwargs)
            except:
                if (requirement.conflicts_with and (not requirement.install_succeeded)):
                    requirement.rollback_uninstall()
                raise
            else:
                if (requirement.conflicts_with and requirement.install_succeeded):
                    requirement.commit_uninstall()
            requirement.remove_temporary_source()
    self.successfully_installed = to_install
