Backport Tim's checkin 2.199: Fix buglet reported on c.l.py: map(fnc, file.xreadlines()) blows up. Took away map()'s insistence that sequences support __len__, and cleaned up the convoluted code that made it *look* like it really cared about __len__ (in fact the old ->len field was only *used* as a flag bit, as the main loop only looked at its sign bit, setting the field to -1 when IndexError got raised; renamed the field to ->saw_IndexError instead).