No 'fold left' without a combiner ?

[Brian Goetz]

> or better, have a foldLeft (or whatever it's name) on Stream that throws
> an IllegalStateException if the stream is parallel.

No, that's not better :)

The pervasive sequential-think that most developers bring to the table 
in 2013 is not anyone's friend.  We've gone to great effort to design an 
API where, if properly used, works correctly regardless of whether the 
pipeline executes sequentially or in parallel.  The result is a cleaner 
and simpler API, and one that better prepares developers for computing 
in the coming decade.

Adding a handful of sequential-only operations may make certain use 
cases easier, but there's a hidden cost -- reasoning about stream 
pipelines becomes harder for everyone, because now parallel 
compatibility becomes an element of the model which must be considered. 
  Short-term gain, long-term pain.  No thank you!

Please deposit your sequential glasses in the bin by the door.  You 
won't be needing them any more.