For instance, what is the example below supposed to print?  And how
are we going to implement this?

    coroutine foo () {
      do {
        temp $x = 2;
        print "$x\n";
        yield;
        print "$x\n";
      }
      print "$x\n";
      yield;
    }

    $x = 0;
    print "$x\n";
    do {
      temp $x = 1;
      print "$x\n";
      foo ();        # This executes foo up to the first yield
      print "$x\n";
    }
    foo ();          # This executes foo from the first yield to the second one
    print "$x\n";

-- Jerome