Utility function for just picking the first from a stream.

Handy when you sometimes want to provide a BinaryOperator and sometimes just want to arbitrarily pick one.
I wonder if "left" might be a better name for this?  Also, if you're going to do "left", I recommend at least also implementing "right". It seems strange to only have one of them, if we're going to put this weird guy into the code.  Also, I'd love to see how you're using this, in a real-world example, because this just seems weird to me.