Improved hold/unhold cascading in Session

Ignore-this: fa577fec85db0b81aeb6fc3156c51565

darcs-hash:20100604105338-1e3d8-d35008d80417d3f27b32eaff0397ef884d845d07
Improved SIPURI with pickling and matching capabilities

Ignore-this: 26db51638db6e2debcef4d32df274433

darcs-hash:20100604105603-1e3d8-d6c4759eb391d617b54625ac42793e1366eeebcd