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;
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end