2010-09-09 ago more abstract treatment of interrupts in structure Exn -- hardly ever need to mention Interrupt literally;
2010-05-29 ago force_result within the current execution context -- avoids overhead of potential thread context switch and robustifies Interrupt handling;
2010-01-06 ago do not memoize interrupts;
2009-10-20 ago removed unused map_force;
2009-10-01 ago separate concurrent/sequential versions of lazy evaluation;