[gsac] Change gsac.prefix.items to be strict in the prefix

Ignore-this: 40908bc7ff7d37324fdff6208565505

darcs-hash:20130620201243-2809a-7be2e83bac1aa56b327b7c7e10c6469e1ba45049
[map] ordered.map.values

Ignore-this: 474b106400ccbac0814baf49f903dd04

darcs-hash:20130620223555-2809a-e4b1c5ecd298f3f301f71a0b6a9b8aadeea08e8e