Add let/par& and define/par& #3
Description
Which would behave like par&, but let the results of the par be bound.
define/par would be a little tricky, since it would need to turn into a nested sequence of of let/par or (or, letrec/par which would be a little tricky), which means it would need to synchronize on when values were ready, which might result in live locks when all sequential versions would raise as use-before-definition error.