added worker_group;
fork: default to subgroup of worker_group;
removed obsolete fork_local;
join/get_result: cumulative flattened exceptions;
map: subgroup of worker_group;
no_document use_thy "Setup";
no_document use_thys ["Efficient_Nat"];
use_thy "Introduction";
use_thy "Program";
use_thy "Adaptation";
use_thy "Further";
use_thy "ML";