?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.au=Butler%2C+Michael&rft.aulast=Butler&rft.aufirst=Michael&rft.issue=4-6&rft.volume=24&rft.date=2012&rft.atitle=External+and+internal+choice+with+event+groups+in+Event-B&rft.title=FORMAL+ASPECTS+OF+COMPUTING&rft.pages=555-567&rft.genre=article<script src='//archive-bar.soton.ac.uk/archive-bar.js'></script>