Binary heap: obtain a _reference_ to the front of the heap

[Johan Engelen via Digitalmars-d-learn]

On Tuesday, 13 September 2016 at 08:19:04 UTC, Johan Engelen 
wrote:
> In the binary heap documentation, I read that 
> `BinaryHeap.front()` "Returns a copy of the front of the heap". 
> [1]
> Is there no function to access the front of the heap without a 
> copy?  (micro-optimization)

Answering myself here: the binary heap element type should be 
cheap to copy, because it is going to be copied around a lot (for 
the common implementation on top of an array).