force_result within the current execution context -- avoids overhead of potential thread context switch and robustifies Interrupt handling; recovered some similarity to sequential version;

future result: retain plain Interrupt for vacuous group exceptions;

remove two examples, now that the definition of "fst" and "snd" has changed

merged

Got rid of a warning about duplicate rewrite rules.

accumulate only local results -- no proper history support yet;

avoid deprecated Iterator.fromArray;