const-correctness in std.range

[Vlad Levenfeld via Digitalmars-d-learn]

I notice that some of the range adapters in std.range (iota, 
takeExactly) have a const empty property, while others (take, 
retro) don't. This makes maintaining const-correctness downstream 
(with my own range adapters atop phobos') more difficult. I'm 
wondering if there is a rationale for this, if there is some use 
for a non-const empty?