protocol for using InputRanges

[w0rp]

I understand it like this.

* empty - Are there no more values?
* front - Get me the current value.
* popFront - Advance to the next value.

In terms of how I implement an InputRange in general, I typically 
end up with this.

* empty - Advance and cache "current value," return true if we 
ran out of values.
* front - enforce(!empty), which in turn caches the current 
value, which we then return.
* popFront - enforce(!empty), clear the cached value so we can 
later advance.

So .front gives you the same thing again and again until you call 
popFront, you could indeed call .front before .empty, but you may 
get an exception. This obviously isn't how I implement all 
InputRanges, as there are better ways to write other ranges, such 
as ranges which operate on sets of integers or wrap arrays. This 
is just my last resort general case InputRange implementation.

.front assignment obviously replaces the cached value.